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Composite Structures
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  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 0263-8223
Published by Elsevier Homepage  [3163 journals]
  • Experimental study on flexural behavior of ECC-concrete composite beams
           reinforced with FRP bars
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Wenjie Ge, Ashraf F. Ashour, Dafu Cao, Weigang Lu, Peiqi Gao, Jiamin Yu, Xiang Ji, Chen Cai This paper presents test results of fifteen reinforced engineered cementitious composite (ECC)-concrete beams. The main parameters investigated were the amount and type of reinforcement, and ECC thickness. All reinforced ECC-concrete composite beams tested were classified into four groups according to the amount and type of main longitudinal reinforcement used; three groups were reinforced with FRP, steel and hybrid FRP/steel bars, respectively, having similar tensile capacity, whereas the fourth group had a larger amount of only FRP reinforcement. In each group, four height replacement ratios of ECC to concrete were studied. The test results showed that the moment capacity and stiffness of concrete beams are improved and the crack width can be well controlled when a concrete layer in the tension zone is replaced with an ECC layer of the same thickness. However, the improvement level of ECC-concrete composite beams was controlled by the type and amount of reinforcement used. Based on the simplified constitutive relationships of materials and plane section assumption, three failure modes and their discriminate formulas are developed. Furthermore, simplified formulas for moment capacity calculations are proposed, predicting good agreement with experimental results.
  • Experimental and numerical investigation of the damping of
           flax–epoxy composite plates
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): S. Mahmoudi, A. Kervoelen, G. Robin, L. Duigou, E.M. Daya, J.M. Cadou In this paper, the vibration behaviour and damping performances of Flax Fiber Reinforced Epoxy (FFRE) are investigated. Static tests using FFRE composite beams are carried out leading to the identification of elastic properties of each layer. Then, three FFRE composite plates are elaborated and used in experimental vibration tests to identify their eigenfrequencies and their modal damping. In numerical part, a constant complex representation of the stiffness is assumed and the loss factors are supposed constants and identified from the first experimental vibration mode. The homogenization technique and the finite element method are applied to describe their damped dynamic behaviour. The resolution of the resulting non-linear problem is carried out using the Asymptotic Numerical Method (ANM). Experimental results show that the modal damping is greater when the flax fibers are oriented at 90°. Comparing numerical and experimental results, the proposed finite element modelling enables to estimate the damped eigenfrequencies with high accuracy. However, the predicted modal loss factors are over-estimated compared to experimental ones. It is concluded that the present modelling should be improved considering the frequency dependence of damping.
  • Viscoelastic response of high volume fraction carbon nanotube-polymer
           nanocomposites with tailored wettability and controlled morphology
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Z. Semih Pehlivan, Deniz Ürk, Hülya Cebeci, M. Lütfi Öveçoğlu, Abdullah Dönmez, Osman Bulut, Fevzi Ç. Cebeci As-grown VACNTs are subjected to mechanical densification by knock-down process to achieve higher volume fractions. In here, the preferential alignment of CNTs is preserved horizontally and an easy delamination of VACNTs are achieved to fabricate high volume fraction nanocomposites. Both physical and chemical properties of VACNTs such as alignment, quality, and purity etc. have been characterized by Raman spectroscopy, TGA, and SEM. The knocked-down VACNTs arrays are then used as ‘reinforcing ply’ with epoxy for PNCs fabrication. Since load transfer in between CNTs and epoxy is important to avoid interfacial slippage and reduction in loss factor, to increase interaction between CNTs and epoxy, ozone treatment was applied to CNTs. To observe the effect of ozone treatment on the viscoelastic response of polymer nanocomposites with non-ozone treated and ozone treated VACNT were tested by DMA under different frequencies. The ozone treatment time and CNT quality, investigated through Raman Spectroscopy, were correlated for viscoelastic properties. The results demonstrated that ozone treatment improved wettability and increased viscoelastic properties of PNCs under multi-frequency for short-term (
  • Explicit dynamic fracture simulation of two-phase materials using a novel
           meso-structure modelling approach
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Yangjian Xu, Shuai Zhao, Guohui Jin, Lihua Liang, Haojie Jiang, Xiaogui Wang In order to reduce the experimental cost and enable the numerical modelling better approaches the actual meso-structures of two-phase materials, a novel meso-scale modelling approach, combining the image-based and the parameterized modelling approaches, has been presented. In this approach, only limited samples of the studied two-phase material need to be provided for establishing an aggregate library, through which finite element models with different spatial distributions and volume fractions of aggregate can be arbitrarily generated for the use of virtual test. In the present virtual test, fracture failures were simulated in the framework of explicit dynamics. A rate-dependent cohesive zone model (CZM) was implemented through the user subroutines in ABAQUS to characterize the dynamic damage and fracture of the meso-structures. At the same time, an algorithm of embedding cohesive elements automatically into the potential damage zones was put forward. Finally, two typical numerical examples were given to verify these proposed methods and models, meanwhile the influences of spatial distribution and volume fraction of aggregate on the mechanical performance of the meso-structure were investigated. It can be validated that the present developed methods and models can effectively and efficiently characterize the dynamic fracture behavior of two-phase materials.
  • Robust topology optimization of multi-material structures considering
           uncertain graded interface
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Zhan Kang, Chunlei Wu, Yangjun Luo, Ming Li Material interface-related uncertainties induced by inter-diffusion or reactions between two different materials may deteriorate the actual performance of a structural design achieved by topology optimization. Thus a rational methodology is needed to address this issue in the design of hybrid-material engineering products implemented by some novel fabrication techniques such as additive manufacturing. This paper presents a robust shape and topology optimization method accounting for uncertain graded interface properties of multi-material structures. A level set function is used to track the evolving material interfaces during the optimization process, and the material interface uncertainties is modeled by introducing an intermediate zone with graded properties represented by a random field. On the basis of discretizing the input random field by means of the Expansion Optimal Linear Estimation (EOLE) method, the uncertain propagation analysis is implemented with the Polynomial Chaos expansion (PCE) to predict the stochastic response. Then the robust shape and topology optimization problem is stated as a multi-criteria optimization problem, in which the expected value and the standard deviation of the performance function of interest are to be minimized under a given material volume constraint. The shape derivative of the stochastic response is derived in the context of Eulerian description, and then used to advance the evolution of the level set function through the Hamilton-Jacobi equation. In the numerical examples, the proposed robust design method is exemplified by the mean compliance minimization problems.
  • On sound insulation of pyramidal lattice sandwich structure
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Jie Liu, Tingting Chen, Yonghui Zhang, Guilin Wen, Qixiang Qing, Hongxin Wang, Ramin Sedaghati, Yi Min Xie Pyramidal lattice sandwich structure (PLSS) exhibits high stiffness and strength-to-weight ratio which can be effectively utilized for designing light-weight load bearing structures for ranging from ground to aerospace vehicles. While these structures provide superior strength to weigh ratio, their sound insulation capacity has not been well understood. The aim of this study is to develop numerical and experimental methods to fundamentally investigate the sound insulation property of the pyramidal lattice sandwich structure with solid trusses (PLSSST). A finite element model has been developed to predict the sound transmission loss (STL) of PLSSST and simulation results have been compared with those obtained experimentally. Parametric studies are then performed using the validated finite element model to investigate the effect of different parameters in pyramidal lattice sandwich structure with hollow trusses (PLSSHT), revealing that the pitching angle, the uniform thickness and the length of the hollow truss and the lattice constant have considerable effects on the sound transmission loss. Finally a design optimization strategy has been formulated to optimize PLSSHT in order to maximize STL while meeting mechanical property requirements. It has been shown that STL of the optimal PLSSHT can be increased by almost 10% at the low-frequency band. The work reported here provides useful information for the noise reduction design of periodic lattice structures.
  • Dynamic response of flexible hybrid electronic material systems
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Nicholas C. Sears, John Daniel Berrigan, Philip R. Buskohl, Ryan L. Harne Flexible hybrid electronic (FHE) material systems embody the intersection of compliant electrical networks and functional material architectures. For a wide variety of future applications, FHE material systems will be subjected to dynamic mechanical stresses, such as for motion monitoring or for vibration isolation. Consequently, an understanding is required on how these new classes of material systems may respond mechanically and electrically when under states of high-cycle and high-frequency loads. Here, conductive silver microflake ink is interfaced with elastomeric geometries programmed with specific strain responses. Changes in electrical resistance under cyclic displacements are shown to depend on the heat generated by electrical current flow and on the thermal heat generation promoted by the pre-strain on the material system. Configurations subject to high static pre-strains and large strain rates exhibit greater increases in temperature and resistance, whereas a near constant conductivity is manifest in FHE material systems with compositions that reduce static local strains despite high engineering pre-strain application. These results may guide future efforts to understand the resistance change in conductive ink networks and expand the use of flexible hybrid electronic material systems into myriad dynamic application environments.
  • Healing efficiency characterization of self-repairing polymer composites
           based on damage continuum mechanics
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): P.S. Tan, A.A. Somashekar, P. Casari, D. Bhattacharyya Self-healing polymer composites are part of a class of materials which have the capability to repair themselves when damaged. They are often tested by inflicting damage onto them and then performing a mechanical test to initiate and measure self-healing. Self-healing efficiency is commonly defined as the ratio of a recovered property value to its original value. Applying this in practice can be challenging as most self-healing experiments are idealized scenarios. In this work, microcapsules containing epoxy and mercaptan self-healing agents were incorporated into ±45° composite weaves that were subjected to cyclic tensile loading and unloading. A damage mechanics theory was applied to quantify the change in stiffness and determine self-healing effectiveness. As this method did not enable a direct comparison between the damaged and healed states of the material, a new methodology was developed to take into account the energy levels of the material in order to quantify self-healing performance.
  • Optimal design of the hard-coating blisk using nonlinear dynamic analysis
           and multi-objective genetic algorithm
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Feng Gao, Wei Sun, Junnan Gao By the nonlinear FE (finite element) analysis and multi-objective genetic algorithm, the optimal design of the integrally bladed disk (blisk) deposited nonlinear hard coating (hard-coating blisk) was investigated in this study. Considering the strain-dependent manner of the hard coating, the nonlinear dynamic modelling and forced vibration analysis of the hard-coating blisk were conducted by the energy-based FE method and the unified iterative calculation based on Newton-Raphson method, respectively. The reference point based NSGA-II (non-dominated sorting genetic algorithm) was used to solve multi-objective optimization problems of the hard-coating blisk, and the optimization results are obtained as Pareto front. Particularly, an academic blisk with NiCoCrAlY + YSZ hard coating was choose as the benchmark to conduct the bi-objective and triple-objective optimization. The results reveal that the designing parameters of the hard coating are constrained mutually, the reference data can be obtained according to the practical requirements, and the specific data located at the “turning point” can balance all aspects of design performance.
  • Special-purpose elements to impose Periodic Boundary Conditions for
           multiscale computational homogenization of composite materials with the
           explicit Finite Element Method
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): S. Sádaba, M. Herráez, F. Naya, C. González, J. Llorca, C.S. Lopes A novel methodology is presented to introduce Periodic Boundary Conditions (PBC) on periodic Representative Volume Elements (RVE) in Finite Element (FE) solvers based on dynamic explicit time integration. This implementation aims at overcoming the difficulties of the explicit FE method in dealing with standard PBC. The proposed approach is based on the implementation of a user-defined element, named a Periodic Boundary Condition Element (PBCE), that enforces the periodicity between periodic nodes through a spring-mass-dashpot system. The methodology is demonstrated in the multiscale simulation of composite materials. Two showcases are presented: one at the scale of computational micromechanics, and another one at the level of computational mesomechanics. The first case demonstrates that the proposed PBCE allows the homogenization of composite ply properties through the explicit FE method with increased efficiency and similar reliability with respect to the equivalent implicit simulations with traditional PBC. The second case demonstrates that the PBCE coupled with Periodic Laminate Elements (PLE) can effectively be applied to the computational homogenization of elastic and strength properties of entire laminates taking into account highly nonlinear effects. Both cases motivate the application of the methodology in multiscale virtual testing in support of the building-block certification of composite materials.
  • Remote line scan thermography for the rapid inspection of composite impact
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): James Moran, Nik Rajic An investigation of the feasibility of inspecting impact damage in polymer composite aircraft components using remote line scan thermography (LST) is presented. A numerical study is undertaken to determine the minimum heat-source beam width required to detect barely visible impact damage (BVID) above a threshold size. It is shown that the beam width requirement can be met with a non-laser-based heating source. A light-weight incandescent focusing-optic luminaire meeting this requirement is designed, developed and experimentally validated. A new image processing strategy for line-scan thermography is introduced. This method, called Dynamic Pulse Phase Thermography (DPPT), does not require static reconstruction of LST data streams and therefore is less restrictive than established processing methods for real-time applications. Experiments on carbon epoxy laminates, one containing flat-bottom-hole defects and another BVID, are used to demonstrate the performance of DPPT compared to Static Pulse Phase Thermography. It is shown that an LST inspection system comprising a light-weight luminaire and a relatively compact low-cost microbolometer, in conjunction with DPPT processing, is able to detect BVID in an aerospace grade carbon-epoxy laminate, at a scan rate of 50 mm/s and a stand-off distance of 500 mm. The significance of this finding for remote LST inspection via an unmanned aerial vehicle is discussed.
  • Impact properties of a new hybrid composite material made from woven
           carbon fibres plus flax fibres in an epoxy matrix
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Zainab Al-Hajaj, Benedict Lawrence Sy, Habiba Bougherara, Radovan Zdero This study is the first to characterize the impact properties of a new hybrid composite made with woven carbon fibres plus unidirectional (i.e. Type A) or cross-ply (i.e. Type B) flax fibres in an epoxy matrix. A pendulum impact tester applied a range of impact energies (5–40 J) to a series composite plates, which were assessed using photographic, thermographic, and geometric techniques. The Type B composite had better impact performance vs the Type A as indicated by lower absorbed energies, higher penetration energy, smaller crack lengths, smaller indentation depths, smaller damage areas, lower temperature rises in the impact zone vs applied impact energy, and higher impact strength. Both the Type A and B hybrid composites had superior impact properties compared to pure flax fibre-reinforced epoxy composites reported in prior literature, suggesting that hybridization using synthetic and natural fibres can be done successfully.
  • Testing of delamination in multidirectional carbon fiber reinforced
           polymer laminates using the vertical eddy current method
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Zhiwei Zeng, Qingze Tian, Handong Wang, Shaoni Jiao, Jian Li This paper studies the testing of delamination in multidirectional carbon fiber reinforced polymer using the vertical eddy current (EC) method. Conditions for producing the EC component perpendicular to the sample surface are investigated. A bridge-type probe including coplanar dual rectangular coils placed perpendicular to the test sample is designed to generate vertical EC component. Where there is delamination that is parallel to the sample surface, the vertical EC is perturbed, which results in an output signal in form of variable differential voltage of the coils. The principle of testing is verified by finite element analysis. Experiments are performed to test the validity of the proposed probe. The results of simulation and experiments show that peaks appear in output signal when the coils cross delamination and the distance between the two peaks of scanning signal can be used to quantitatively estimate the range of delamination.
  • Finite element investigation of the fatigue performance of FRP laminates
           bonded to concrete
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): A. Al-Saoudi, R. Al-Mahaidi, R. Kalfat, J. Cervenka The effectiveness of using FRP materials to strengthen existing concrete structures is hampered by the weak bond properties between the FRP and the concrete. Further, very few studies are available on the fatigue performance of FRP-strengthened reinforced concrete beams in bridges. Investigation of the fatigue performance of FRP bonded to concrete is important due to the fact that bridges are subjected to very high levels of cyclic loading throughout their lifetime. This paper focuses on a numerical and experimental investigation of the fatigue life of FRP laminates bonded to concrete. FRP-to-concrete joints were subjected to cyclic loading at various stress ratios and the number of cycles prior to failure was used to generate S-N curves which relate the stress ratio to the number of cycles. Further, numerical simulations using the finite element method were performed using a concrete material model capable of sustaining fatigue damage. The finite element results were found to be in good agreement with the experimental data and were used to provide further insights into the mechanisms of fatigue damage. Both the experimental and numerical results showed that no fatigue degradation occurred when the maximum stress ratio was less than 75% of the ultimate static capacity.
  • Micromechanical modelling of matrix cracks effect on shear and transverse
           response for unidirectional composites with a full field approach
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): B. Burgarella, A. Maurel-Pantel, N. Lahellec, C. Hochard Modelling the damage of composite materials is not an easy task because different modes of ruins coexist: Fiber matrix decohesion,matrix cracks, delamination, and fiber cracks. In the case of laminated composites, the matrix cracks have the particularity to remain parallel to the fibers. As a consequence of the orientation of this crack network, only shear and transverse moduli in the plane of the ply are degraded in proportion to the increase of the crack density. The main point of this work is to characterize the relation linking transverse and shear damage with respect to the crack density. Following this objective, full field calculations are run using CraFT, a software developed at the LMA. The modeling is done in two steps: first the undamaged composite is homogenized, then, as a second step, the damaged behavior is determined by introducing cracks into the healthy composite. The behavior is calculated from an optimal size of RVE (Representative Volume Element) in order to determine numerically the relation between transverse and shear moduli variables.
  • A modeling method for vibration analysis of cracked laminated composite
           beam of uniform rectangular cross-section with arbitrary boundary
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Kwanghun Kim, Kwangnam Choe, Sok Kim, Qingshan Wang This paper establishes an analysis model to study the vibration behavior of a cracked laminated composite beam with uniform rectangular cross-section based on the Jacobi-Ritz method and the first-order shear deformation theory (FSDT). The boundary conditions of both ends of the cracked laminated beam are modeled as the elastic spring and the beam is divided into two parts by the crack section. The continuous conditions at the connecting face are modeled by the inverse of the flexibility coefficients of the fracture mechanics theory. Ignoring the influence of boundary conditions, displacements admissible functions of cracked laminated beam can be set up as Jacobi orthogonal polynomials. The accuracy and robustness of the present method are evidenced through comparison with previous literature and the results achieved by the finite element method (FEM). Numerical examples are given for free vibration analysis of cracked laminated composite beams with various boundary conditions, which may be provided as reference data for future study.
  • A valid inhomogeneous cell-based smoothed finite element model for the
           transient characteristics of functionally graded magneto-electro-elastic
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Liming Zhou, Shuhui Ren, Changyi Liu, Zhichao Ma For the sake of surmounting defect of over-stiffness of finite element model (FEM) and accurately solving the transient response problems of structures comprise functionally graded magneto-electro-elastic (FGMEE) materials, we put forward an inhomogeneous cell-based smoothed finite element model (ICS-FEM) and a modified Newmark method. By employing the inhomogeneous gradient smoothing technique into FEM, the mass matrix M and the equivalent stiffness matrix Keq are derived, ICS-FEM that provides a stiffness coinciding with the actual condition is also obtained. Moreover, this model can be carried out with user-defined subroutines in the existing FEM software. Several numerical examples including cantilever beams, a layered FGMEE sensor and an FGMEE energy harvester are analyzed, which prove that ICS-FEM could achieve results with higher accuracy and reliability than FEM. ICS-FEM are applied to more complex structures such as FGMEE layered sensor and energy harvester. Therefore, such method to solve the transient characteristics of FGMEE structures can be a reference for the design of smart structures.
  • Thermo elastic- up to yielding behavior of three dimensional functionally
           graded cylindrical panel based on a full layer-wise theory
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): M. Khakpour Komarsofla, S. Jedari Salami, M. Shakeri As a first endeavor, the full layer-wise method is presented for elasto-up to yielding analysis of a functionally graded cylindrical panel with different boundary conditions and under steady state thermal and nonuniform mechanical loads. Static load and thermal field are applied in large and nonuniform scales which are acted on the inner and outer layers of the panel. Tresca and von Mises yield criteria are compared together and with TTO (Tamura–Tomota–Ozawa) model to find the yielded regions in the panel. The results of the used method are compared with a simulation using the commercial ABAQUS. The stress and temperature fields were expressed on the basis of an analytical solution using trigonometric terms for the full simply supported boundary conditions and simply-free boundary conditions and the derived results are presented graphically. Yielded areas vary by changing the geometry of boundary conditions and are closer to the supports. Both von Mises and Tresca criteria predicted same yielded region with the distinction that von Mises criterion is more conservative compared to Tresca criterion. Yield modes vary in different layers of the panel through its thickness and according to von Mises and Tresca yield criteria and the TTO model in this analysis, yield begins from the inner layer of the panel. By increasing the temperature, yield stress in the panel increased and by increasing FGM index and thickness of the panel, yield stress decrease in the panel.
  • Vibration control and analysis of a rotating flexible FGM beam with a
           lumped mass in temperature field
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Liang Li, Wei-Hsin Liao, Dingguo Zhang, Yang Zhang Based on the high-order coupling (HOC) modeling theory, vibration control of a rotating rigid-flexible coupled smart composite structure in temperature field is investigated. A flexible beam made of functionally graded materials (FGM) with a lumped mass and two piezoelectric films perfectly bonded to it is attached to a horizontal rotating hub. By using the method of assumed modes to describe the deformations of the FGM beam and piezoelectric films, the rigid-flexible coupling dynamic equations of the system with the high order geometric nonlinear terms are derived via employing Lagrange’s equations. A PD controller is used in the vibration control of the system. Simulation results indicate that the intense thermally induced vibrations of the FGM beam along the longitudinal and transverse direction are efficiently suppressed after the piezoelectric active control effect works. The HOC model is more accurate than the previous low order coupled (LOC) model when the temperature gradient increases. The influence of high-order nonlinearity in the present HOC model on the characteristics of dynamics and control of flexible structures should not be ignored. The effect of temperature variation on the free vibration characteristics of the rotating smart structure is gentle despite non-negligibility.
  • Multi-scale progressive failure simulation of 3D woven composites under
           uniaxial tension
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Gang Liu, Li Zhang, Licheng Guo, Feng Liao, Tao Zheng, Suyang Zhong This paper presents a multi-scale progressive failure modeling scheme to analyze the damage behaviors of 3D angle-interlock woven composites under uniaxial tension. The macro-scale progressive damage model is established based on a meso-scale representative volume cell (RVC) model by using the inhomogeneous finite element method. In current model, a modified Puck criterion for fiber yarn and parabolic yield criterion for the matrix are chosen to be the damage initiation and propagation criteria, which can clearly describe the fiber breakage, inter-fiber fracture and matrix crack in the level of the fiber yarn and the matrix. The tensile effective elastic properties and the failure strength as well as the damage evolution process of this 3D woven composite are predicted. A series of uniaxial tensile tests are conducted to validate the macro-scale progressive damage model. Experimental and numerical results are compared and discussed.
  • X-band microwave characterisation and analysis of carbon fibre-reinforced
           polymer composites
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Zhen Li, Arthur Haigh, Constantinos Soutis, Andrew Gibson In this paper, a detailed investigation of the electromagnetic (EM) properties of carbon fibre-reinforced polymer composites (CFRP) is presented. The electric permittivity, electrical conductivity, signal penetration and microwave absorption are discussed. Unidirectional composite laminate samples were characterised over X band (8–12 GHz) using the microwave transmission line technique. It is shown that the real part of the permittivity of the composites is not significantly anisotropic whereas the imaginary part is highly anisotropic. More microwave energy is reflected in the parallel case, while more energy is absorbed in the orthogonal case. These experimental results are studied at three geometric levels: macro-meso scale (laminate/lamina level) relating to lay-up and fibre direction dependence, microscale (fibre level) relating to the real part of the permittivity and nanoscale level relating to the imaginary part of the permittivity. The findings can contribute to improved design of carbon fibre composites for electromagnetic applications, like shielding, curing and non-destructive inspection.
  • Experimental and numerical analysis of wrinkling during forming of
           multi-layered textile composites
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): E. Guzman-Maldonado, P. Wang, N. Hamila, P. Boisse Wrinkling is one of the main flaws that can appear during forming processes of textile composite reinforcements. When the forming is carried out on a multi-layered fabric the wrinkle development is strongly increased if the plies have different orientations. This phenomenon has been observed in previous experimental studies and is confirmed by a set of forming tests on multi-layered reinforcements. Beyond these experiments, a multilayered fabric forming process numerical simulation based on stress-resultant textile shell elements is presented. It is shown that this simulation is able to accurately describe multi-layered fabric forming processes and in particular the development of wrinkles. In the case of multi-layered reinforcements where the neighboring plies are oriented differently, the numerical simulations shows the development of zones where the fibers in one direction are subjected to compression which gives rise to wrinkles. The analysis of the forming for different friction coefficients confirms the major role of the friction between the plies in wrinkling. The influence of the pressure imposed by the blank holder has also been studied.
  • Mechanical behavior analysis of 3D braided composite joint via experiment
           and multiscale finite element method
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Yang Hong, Ying Yan, Ziyang Tian, Fangliang Guo, Jinxin Ye This study investigates the mechanical behavior of 3D braided composite truss joints through experiments and a multiscale finite element method. Unit cell models of 3D braided composites, which take into account the yarns, matrix and interface, are established to predict the mechanical properties. A novel damage model based on a modified Tsai-Wu criterion is developed to characterize the progressive damage of the composites. Excellent loading capacities of the joints suggest that 3D braided composites are applicable to complex structural components. The simulation results of the load-displacement curve, strain process, and the damage phenomenology are in good agreement with experimental data. This study also discusses the influence of braiding angle and fiber volume fraction on the loading capacity. The decrease in braiding angle and increase in fiber volume fraction of the composites can improve the loading capacity of the joints.
  • A hybrid approach for global sensitivity analysis of FRP composite
           multi-bolt joints
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Bibekananda Mandal, Souvik Chakraborty, Anupam Chakrabarti This paper presents a hybrid approach for global sensitivity analysis of FRP composite multi-bolt joints. The proposed approach integrates a spring-based analytical model with moment-independent sensitivity analysis. The homogenized properties within the spring-based analytical model are computed by utilizing a novel homogenization technique for FRP composites. The proposed model has been utilized for stochastic sensitivity analysis of a generic double-lap multi-bolt FRP composite joint where, both material and geometric properties are uncertain. The importance of the input parameters with respect to structural responses of the bolted joint has been computed. The proposed approach is found to be highly efficient, even for realistic composite model with significant number of layers. This indicates that the proposed model can be used to save computational time and design effort by identifying the most important design parameters.
  • Joining of carbon fiber and aluminum using ultrasonic additive
           manufacturing (UAM)
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Hongqi Guo, M. Bryant Gingerich, Leon M. Headings, Ryan Hahnlen, Marcelo J. Dapino Various methods have been reported to join carbon fiber reinforced polymer (CFRP) composites with aluminum alloy (AA), with strengths ranging from 13 MPa to 112 MPa. This paper presents a new method for joining carbon fiber reinforced composites and metals using ultrasonic additive manufacturing (UAM). Although UAM is a metal 3D printing process, it is applied here to produce continuous CF-AA transition joints that can have uniform thickness across the CF and AA constituents. Joint strength is achieved by mechanical interlocking of CF loops within the AA matrix; tensile tests demonstrate that UAM CFRP-AA joints reach strengths of 129.5 MPa. The dry CF fabric extending from these joints can be laid up and cured into a CFRP part, whereas the AA can be welded to metal structures using traditional metal welding techniques – hence their designation as “transition joints.” This approach enables the incorporation of CFRP parts into vehicle structures without requiring modifications to existing metal welding infrastructure. Two failure modes, CF tow failure and AA failure, have been identified. It is shown that the joint failure mode can be designed for maximum strength or maximum energy dissipation by adjusting the ratio of embedded CF to AA matrix.
  • A mixed edge-based smoothed solid-shell finite element method (MES-FEM)
           for laminated shell structures
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Leonardo Leonetti, H. Nguyen-Xuan We in this study present a mixed solid-shell finite element formulation for laminated shell analysis. With the commonly adopted truncations applied to the strain components and the Jacobian determinant, a generalized stiffness matrix is established. Discrete shear gap (DSG) technique together with assumed natural strain (ANS) method is employed in order to alleviate shear locking and trapezoidal locking. Stiffness matrix formulation is analytically calculated based on the edge-based cell backgrounds, which result in simple and efficient computation. The proposed formulation does not require using cross-diagonal meshes, which can be an obstacle for several existing three-node degenerated shell elements. This aspect makes the present method much more appealing than others through examples tested. Numerical validations show that the present method performs well for both structured and unstructured triangular meshes.
  • Size- and edge-effect cohesive energy and shear strength between graphene,
           carbon nanotubes and nanofibers: Continuum modeling and molecular dynamics
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Yinfeng Chen, Dongqing Ding, Chunhua Zhu, Junhua Zhao, Timon Rabczuk Explicit expressions for the size- and edge-effect cohesive energy and shear stress between two finite-sized graphene (graphene/graphene), two finite-sized carbon nanotubes (CNTs) (CNT/CNT) and two finite-sized nanofibers (nanofiber/nanofiber) are obtained through continuum modeling of van der Waals (vdW) interactions between them. The close-form solutions of the cohesive energy and shear stress between these structures at different positions are derived by using Gaussian quadrature. The analytical results of the edge-effect cohesive energy show that both of the maximum repulsive and attractive shear stresses are always close to the initial intersecting positions between them. Checking against present molecular dynamics (MD) calculations and available experimental results shows that the continuum solutions are reasonable, in which the main reason of their difference is also revealed in detail. The obtained analytical solutions should be of great help for understanding the size- and edge-effect interactions between these nanostructures and designing nanoelectronic devices.
  • Acoustic emission monitoring of damage progression in 3D braiding
           composite shafts during torsional tests
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Wenfeng Hao, Zengrui Yuan, Can Tang, Lu Zhang, Guoqi Zhao, Ying Luo In this work, damage progression in 3D braiding composite shafts during torsional tests was monitored using the acoustic emission (AE) method. The 3D 4-directional braided composite shafts with various braiding angles were prepared using Vacuum Assisted Resin Transfer Moulding (VARTM). The torsional tests were performed on the static torsion machine and the AE method was used to monitor the damage progression. The continuous wavelet transform method was used to analyze the AE signals for time-frequency analysis. The accumulated damage is manifested via matrix cracking, fiber bundle-matrix debonding, and fiber breakage, which were identified by the frequency of the AE signals. The failure modes and the mechanisms of the 3D 4-directional braided composite shafts were characterized. The results will enable the design and the application of the 3D 4-directional braided composite shafts within engineering.
  • A novel implementation of asymptotic homogenization for viscoelastic
           composites with periodic microstructures
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Quhao Li, Wenjiong Chen, Shutian Liu, Jiaxing Wang There is a growing demand for methods to estimate the effective viscoelastic response of viscoelastic composites, for their applications in structural vibration and noise control. This paper proposes a novel reformulation and numerical implementation algorithm for the asymptotic homogenization theory for predicting the effective complex moduli of viscoelastic composites in the frequency domain. In the new algorithm, an equivalent harmonic analysis is established and a double-layer elements method is proposed to solve the local problem in the homogenization process. On the basis of the new algorithm, the effective complex moduli can be obtained easily by using commercial software to serve as a black box. Numerous elements and techniques for modeling and analysis available in commercial software can be applied to complicated microstructures without mathematical derivation. The numerical examples presented show the validity of this new implementation algorithm.
  • Monitoring the drilling process of GFRP laminates with carbon nanotube
           buckypaper sensor
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Gong Dong Wang, Nan Li, Stephen Kirwa Melly, Tian Peng, Ying chi Li, Qi Di Zhao, Shu De Ji The present study presents an experimental investigation into the real-time monitoring of the drilling process of Glass Fiber Reinforced Polymer (GFRP) composite with Carbon Nanotubes (CNTs) buckypaper sensor. The spray-vacuum filtration method was employed to synthesize CNTs buckypaper sensor and then embedded into the GFRP to serve two purposes namely interlaminar enhancement and sensor monitoring the drilling process of the GFRP. Both modes I and II interlaminar fracture toughness tests were conducted to ascertain the enhancement ability of the buckypaper. Considerable improvements in both modes of fracture toughness were recorded in the specimens with CNTs buckypaper interlayer. For the CNTs buckypaper acting as a sensor, drilling experiments were carried out on the GFRP where real-time information of the drilling process particularly the drilling tool position could be predicted. This can be crucial in cases where drilling parameters like the feed rate have to be changed at a certain drilling depth in order to reduce or eliminate the exit delamination. The Scanning Electron Microscope (SEM) was finally employed to study the enhancement ability of the CNTs buckypaper and the microstructure of the drilled holes. The current work has substantiated the possibility of using CNT buckypaper both as an interlaminar enhancement and as a sensor to monitor the drilling process.
  • A robust and reliability-based aeroelastic tailoring framework for
           composite aircraft wings
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Muhammad F. Othman, Gustavo H.C. Silva, Pedro H. Cabral, Alex P. Prado, Alberto Pirrera, Jonathan E. Cooper This paper presents a multi-level aeroelastic tailoring framework for the optimisation of composite aircraft wings. The framework is capable of structural sizing and produces detailed composite ply configurations through robust and reliability-based design optimisation, and is demonstrated on a representative regional jet airliner finite element wing box model. The optimisation procedure is divided into two levels. The first level optimises the wing structure for minimum weight subject to multiple constraints including strain, buckling, aeroelastic stability and gust response. These first level solutions are then fed into the second level to be further optimised for robustness or reliability by considering uncertainties in material properties at ply level. Both the principles of robust and reliability-based design optimisation can also be used in combination to ensure a balance between the robustness and reliability of the structural performance. In order to keep computations to an acceptable cost, the second level optimisation employs the Polynomial Chaos Expansion method to approximate the effect of probabilistic uncertainty on structural performance. In comparison to the original benchmark wing, the framework produces an overall weight reduction of 32.1%, despite a 1.5% increase from the first to the second level optimisation that accounts for stochastic design variations.
  • New transverse shear deformation theory for bending analysis of FGM plate
           under patch load
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Rahul Kumar, Achchhe Lal, B.N. Singh, Jeeoot Singh Two new higher order transverse shear deformation theories (NHSDTs) with five variables have been proposed for the analysis of Functionally Graded Material (FGM) plate. A Governing differential equation (GDE) of the FGM plate is developed using energy principle. Wendland radial basic function (RBF) based Meshfree method is implemented for discretizing the GDE. A MATLAB code is developed to obtain the desired results. The normalized deflection and stresses are obtained and compared with other published results. The effect of different types of load, span to thickness ratio, grading index on the response is studied. Some new results for patch loading are also obtained.
  • Vibration characteristics of a rotating pre-twisted composite laminated
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Jie Chen, Qiu-Sheng Li A new dynamic model based on the shell theory is presented to investigate the vibration behavior of a rotating composite laminated blade with a pre-twisted angle. The effects of the Coriolis and centrifugal forces due to the rotation motion of the blade are considered in the formulation. Based on the Rayleigh-Ritz method and continuous algebraic polynomial functions satisfying the boundary conditions of a cantilever, the natural frequencies and mode shapes of a rotating pre-twisted blade are obtained. The convergence analysis is performed and the accuracy of the proposed model is verified by comparing the non-dimensional frequencies obtained by the present method with those in literature. The frequency loci veering and crossing phenomena along with the corresponding mode shape variations are presented and discussed in detail. A comprehensive parameter investigation of the effects of aspect ratio, pre-twisted angle, stagger angle, rotation velocity and hub radius on variations of the modal characteristics of the blade is conducted. It is demonstrated through the results of this paper that the developed model is effective to evaluate the dynamic behavior of rotating pre-twisted blades, which would be useful for improvement in design and optimization of the material and geometry dimension of the blades.
  • Mechanical behaviour of glued-in rods (carbon and glass fibre-reinforced
           polymers) for timber structures—An analytical and experimental study
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): M. Titirla, L. Michel, E. Ferrier We address the mechanical response of glued in rods, taking into account parameters such as rod diameter, the material properties of the rods (carbon or glass fibre-reinforced polymer), and the grain orientation of the timber (rods parallel and perpendicular to the timber grain). All the experimental setups employ pull-out tests, where rods are glued in only on one side of the specimen, and these tests are assessed at the premises of the Laboratory for Composite Materials and Composite Structures at the University Claude Bernard Lyon I. In addition, we give an overview and present known design models, technical approvals and regulations, national standards, and guidance papers, comparing the different approaches among them and with the experimental results. Good agreement was shown and further useful results were observed.
  • Performance of precast segmental concrete beams posttensioned with carbon
           fiber-reinforced polymer (CFRP) tendons
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Tan D. Le, Thong M. Pham, Hong Hao, Cheng Yuan Precast segmental prestressed concrete beams (PSBs) have been widely used in many elevated highway bridge projects around the world. Steel tendons at joint locations, however, are vulnerable to corrosion damages, which cause deteriorations and in extreme cases lead to the collapse of the whole structures. This study experimentally investigates the use of carbon fibre-reinforced polymer (CFRP) tendons as an alternative solution for the PSBs to tackle the corrosion issue. Four large-scale segmental T-shaped concrete beams with internal bonded or unbonded tendons and dry or epoxied joints were built and tested under four-point loading. The test results indicated that CFRP tendons showed satisfactory performances therefore could replace steel tendons for the use in PSBs. All the tested beams exhibited excellent load-carrying capacity and ductility. Tendon bonding condition greatly affected the flexural performance of the segmental beams. Joint type had only a slight effect on the load-carrying capacity and ductility of the beams, but significantly affected the beams’ initial stiffness. Unbonded tendons experienced an evident reduction in the tendon strength at the ultimate stage as a consequence of the loading type, harping effect and joint opening. Both AASTHO-1999 and ACI 440.4R-04 predicted well the tendon stress, thus the load-carrying capacity of the beams with bonded tendons, however, the accuracy significantly reduced for the cases with unbonded tendons. Similarly, the codes did not well estimate the deformation capacity of the prestressed beams with unbonded tendons. An empirical formula is proposed to predict the deflections of beams with unbonded tendons, which yields very close predictions to the experimental results.
  • Off-axis bending behaviors and failure characterization of 3D woven
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Diantang Zhang, Mengyao Sun, Xiaodong Liu, Xueliang Xiao, Kun Qian This paper presents the influence of the off-axis angles on the flexure behaviors of three-dimensional (3D) woven carbon/epoxy composites. Four kind of samples with different angles, 0 degree, 30 degree, 45 degree and 90 degree, are experimentally tested via three-point bending method. The immersion focused ultrasound imaging and micro-computed tomography (Micro-CT) techniques are employed to investigate the deformation and damage mechanisms in the specimens fractured in bending. Results indicate that the stress-deflections of on-axis (0 degree and 90 degree) samples exhibit obvious quasi-brittle behaviors, whereas those of off-axis (30 degree and 45 degree) samples show important ductile features. Furthermore, the dominant failure mechanism of on-axis samples are identified to be inter-ply delamination and fiber bundle fractures, whereas those of off-axis samples are matrix cracks and tows debonding.
  • The elastic properties of composites reinforced by a transversely
           isotropic random fibre-network
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Xiude Lin, Hanxing Zhu, Xiaoli Yuan, Zuobin Wang, Stephane Bordas This research stems from the idea of introducing a fibre-network structure into composites aiming to enhance the stiffness and strength of the composites. A novel new type of composites reinforced by a tranversely isotropic fibre-network in which the fibres are devided into continuous segments and randomly distributed has been proposed and found to have improved elastic properties compared to other conventional fibre or particle composites mainly due to the introduction of cross-linkers among the fibres. Combining with the effects of Poisson’s ratio of the constituent materials, the fibre network composite can exhibit extraordinary stiffness. A simplified analytical model has also been proposed for comparison with the numerical results, showing close prediction of the stiffness of the fibre-network composites. Moreover, as a plate structure, the thickness of the fibre network composite is adjustable and can be tailored according to the dimensions and mechanical properties as demanded in industry.
  • A numerical study of variability in the manufacturing process of thick
           composite parts
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): M.Y. Matveev, J.P.-H. Belnoue, O.J. Nixon-Pearson, D.S. Ivanov, A.C. Long, S.R. Hallett, I.A. Jones Consolidation of a prepreg layup to a target thickness is critical in order to achieve the required fibre volume fraction and dimensions in a composite part. Experiments show that different processing conditions lead to different levels of compaction and variability in the thickness. This paper presents an analysis of processing conditions and their effects on consolidation of thick composite components. A model that accounts for both percolation and squeezing flow is employed to study two toughened prepreg systems – IM7/8552 and IMA/M21. This paper analyses the significance of the process parameters on the thickness of prepregs and its variability. The analysis of different layups and processing conditions suggests several strategies to control target thickness and its variability. The IMA/M21 prepreg system was found to have lower variability due to its toughening mechanism. The presented results provide a better understanding of the composite manufacturing and can be used to provide an informed choice in design for manufacture of composite structures.
  • Interlaminar properties of GFRP laminates toughened by CNTs buckypaper
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Nan Li, Gong dong Wang, Stephen Kirwa Melly, Tian Peng, Ying Chi Li, Qi Di Zhao, Shu de Ji The interlaminar property is essential to the application of fiber reinforced composites for the overall performance. For the purpose of enhancing the interlaminar properties, the unidirectional Glass Fibre Reinforced Plastics (GFRP) laminates were fabricated by adding carbon nanotubes (CNTs) buckypaper to the mid-plane layers under different curing pressures (1 MPa, 2 MPa, 3 MPa). In order to study the enhancement mechanism of the CNTs buckypaper, a series of experiments were designed and conducted based on double cantilever beam (DCB), end notch flexure (ENF) and interlaminar shear strength (ILSS) tests followed by scanning electron microscope (SEM) examination to study the surfaces of the specimens. It was observed that the modes I and II interlaminar fracture toughness of GFRP embedded with CNTs buckypaper were relatively higher than those without CNTs buckypaper. Considerable improvements in interlaminar shear strength of the laminates were obtained with the integration of the CNTs buckypaper between the interlayers. The micro-structure of the enhanced mixed interface of fiber-resin-CNTs responsible for interlaminar strengthening was further studied with the help of SEM. It was found that the higher curing pressure could increase the inter-diffusion of the resin and CNTs offering a stronger interfacial adhesion. The interlaminar characteristics of the specimens cured at 2 MPa pressure were found to be relatively superior with optimal interlaminar fracture toughness and interlaminar shear strength.
  • Structural design and manufacturing process of a low scale bio-inspired
           wind turbine blades
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): Camilo Herrera, Mariana Correa, Valentina Villada, Juan D. Vanegas, Juan G. García, César Nieto-Londoño, Julián Sierra-Pérez A wind turbine blade design inspired by a tree seed called Triplaris Americana is presented. The blade was designed by means of an analysis of the seed’s curvature and airfoil along its wingspan; the result is as a non-conventional horizontal axis wind turbine composed of three blades. A computational fluid dynamic simulation was performed in order to estimate the operational loads. The blade’s structure was designed by means of composite structural design, resulting in six zones with different laminates of carbon fiber. The balance of the aerodynamic and inertial loads was achieved in order to guarantee a minimum change in blade’s geometry to prevent a performance reduction. Finally, a manufacturing simulation by means of vacuum assisted resin infusion was performed. Four injections strategies were proposed with three of them considered successful based on a complete mold filling and the time limit imposed by the polymerization time of the resin.
  • Structure-preserving low-order modeling approach of laminated composite
           plates integrated with macro-fiber composite transducers for dynamic
    • Abstract: Publication date: 15 January 2019Source: Composite Structures, Volume 208Author(s): ZhongZhe Dong, Cassio Faria, Bert Pluymers, Martin Hromčík, Michael Šebek, Wim Desmet An Equivalent Substructure Modeling (ESM) approach of laminated composite plates with integrated macro-fiber composite (MFC) transducers is described. The proposed approach generates structure-preserving low-order system models for dynamic applications such as vibration suppression and energy harvesting. The direct piezoelectric effect of MFC transducers is derived from the electrical boundary conditions and it has the same coupling patterns as equivalent forces which describe the inverse piezoelectric effect. Hence, the reversibility of the piezoelectric effect is ensured and the electrical dynamics of the system can be simulated. The dual electromechanical couplings are assigned to a low-order structural model generated by an equivalent substructure concept. A laminated composite plate with integrated MFC transducers is used for validations and the simulated results agree well with experimental data. Two given study cases demonstrated that the ESM approach not only can generate accurate low-order system models but also provides a flexible fashion to design piezoelectric composite systems.
  • Thermo-mechanical modelling of laminated glass with the use of
           two-dimensional in-plane mesh
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): P. Pluciński, J. Jaśkowiec The three-dimensional (3D) numerical modelling of laminated glass (LG) plate subjected to coupled thermomechanical loading is in the scope of this paper. The method called FEM23 is applied, in which a 2D in-plane mesh is used, however full 3D results are obtained. In any LG plate glass panes are bonded by very thin polymer films. This layered structure consists of subsequent thick and thin layers of glass and polymer, respectively. Additionally, the thermal and mechanical properties of the glass and the bonding polymer are significantly different. FEM23 is suitable for analyses of such kind of structure. The full 3D results of the coupled problem are obtained following special FEM23 postprocessing. FEM23 is a relatively simple, robust and effective method and 3D thermo-mechanical results obtained are correct for both stationary and non-stationary heat transport. The accuracy of the method has been examined with the use of solutions obtained from the ABAQUS system. The examples presented in the article include two-, three- and four-paned LG plates.
  • Investigation of fast curing epoxy resins regarding process induced
           distortions of fibre reinforced composites
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Fabian Groh, Erik Kappel, Christian Hühne, Wojciech Brymerski Unavoidable deformations occur during part production due to the non-isotropic nature of carbon fibre reinforced plastics (CFRP). These deformations often lead to dissatisfaction of tolerances or result in cost and time intensive rework of the tooling. In a cost driven production environment, similar to the automotive industry, it is essential to predict the deformations early on in the part development process in order to compensate toolings accordingly. In future applications, Fast Curing Epoxy Resins (FCER), with curing times of less than 20 min, will play a key role in high rate CFRP-production at low cost.The present paper reports on a comprehensive experimental study on different FCER systems. It includes the thermo-chemical characterization of neat resin samples as well as the quantification of spring-in deformations of L-profiles. Essential part and processing parameters, as the lay-up, the curing temperature, the cooling rate and the fibre volume fraction are varied and their effect on process induced deformations is quantified. Results for FCER system are compared to slower curing systems to assess differences.
  • Optimal design of fighter aircraft wing panels laminates under multi-load
           case environment by ply-drop and ply-migrations
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Sachin Shrivastava, P.M. Mohite, M.D. Limaye The ply-drop (PD) is termination of specific plies at rib-axis for getting tapered laminates. The present optimization study aims to achieve minimum weight tapered wing panels laminates by PD followed by ply-migrations (PM). The PM are required for ply-continuity (blending) and achieving smooth external aerodynamic surface. A genetic-algorithm mutation operator and fitness based search algorithm is developed in the present study for the optimization. The laminate weight minimization has been achieved as goal of multi-objective optimization (MOO), by utilizing excess design margins of Tsai-Wu first ply failure-index (FI) and wing tip lateral deflection. The finite-element (FE) model of laminate is a set of discrete laminates (chromosomes) between ribs with continuity by virtue of ply-orientations. To select best fit laminate, ply orientations were randomly selected and perturbed for thickness during optimization. The fitness function for evaluating chromosomes is a composite function of multi-objective design requirements and design constraints. The algorithm submits orientation/thickness combinations to ABAQUS/CAE by python-script for function evaluation. The application of algorithm over an initially assumed quasi-isotropic laminate of uniform thickness showed 57% weight reduction for a fighter aircraft’s wing panel. The optimization process is automated making PD practically viable in the design process itself.
  • Prediction of in situ strengths in composites: Some
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): G. Catalanotti The classic formulation that relates the fracture toughness with the in situ strength of the ply implicitly assumes that a fracture process zone fully develops within the ply. This assumption, reasonable for conventional composite laminates, may not be appropriate when the ply thickness is very small or the fracture process zone very large. In the following it is shown how considering the R-curve of the material, the in situ strength for the cases when the fracture process zone cannot develop completely can be correctly computed. Closed form solutions are found for the in situ strengths, and for their maximum values that are obtained when the ply thickness approaches zero.
  • Comparison of buckling loads of hyperboloidal and cylindrical lattice
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Atsushi Shitanaka, Takahira Aoki, Tomohiro Yokozeki Cylindrical lattice structures are types of lightweight structures and can utilize the orthotropy of fiber-reinforced composite materials. Maximization of the buckling loads is important because they dominate the strength of the structures. On the basis of the fact that a hyperboloidal shape deviation increases the buckling loads of cylindrical homogeneous shells, the changes in the buckling loads of cylindrical lattice structures with a hyperboloidal shape deviation are discussed. Further, the buckling loads of both conventional cylindrical lattice structures and shape-deviated hyperboloidal lattice structures with respect to the compressive, bending, and torsional loads are calculated using a finite element method. The results show that the shape deviation decreases the buckling loads, while the change in mass is negligible. The effects of the shape deviation on the buckling loads differ between homogeneous shell structures and lattice structures.
  • Effect of initial imperfections of struts on the mechanical behavior of
           tensegrity structures
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Jianguo Cai, Ruiguo Yang, Xinyu Wang, Jian Feng Tensegrity has been widely accepted as a conceptual model for cell mechanics. Tensegrity models developed to study cell mechanical behavior have never considered the initial imperfection of the struts. However, the curved shapes of microtubules in living cells and the nonlinear cell mechanical behavior lend support to the necessity of introducing initial imperfections to cell tensegrity. Here we use a simple three-member tensegrity structure to investigate the mechanical behavior of tensegrity structures with initial imperfections. Our analytical results demonstrated that the stiffness of the tensegrity structure will be reduced significantly with even small initial imperfections. Further, a strong nonlinearity arises with increased initial imperfection both in terms of the maximum deformation of the strut as well as the stiffness behavior of the tensegrity. Finite element simulations of tensegrity structures with 2 struts and 3 struts show great consistency with the analytical solution. The initial imperfection method provides an intuitive way to introduce nonlinearity to cellular mechanical behavior. It has major implications in understanding load bearing capacity and force distribution in cytoskeleton.
  • 3D stochastic computational homogenization model for carbon fiber
           reinforced CNT/epoxy composites with spatially random properties
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Sungwoo Jeong, Feiyan Zhu, Hyoungjun Lim, Yeonghwan Kim, Gun Jin Yun In this paper, a 3D stochastic computational homogenization model for carbon fiber-reinforced (CFR) CNT/epoxy matrix composites was presented. Stochastic waviness, agglomeration and orientation of CNT fillers cause random spatial variations of the elasticity tensor of the CNT/epoxy matrix within a microscale RVE, resulting in probabilistic variations of the effective homogenized stiffness of the RVE. The present computational homogenization model is based on two-scale asymptotic homogenization theory. An ensemble average of the multiple homogenized effective stiffnesses was obtained for structural analysis of the macroscale CFR CNT/epoxy composite materials. From the proposed model, we could observe significant effects of the CNT alignment orientations, agglomeration and waviness on the effective stiffnesses. Effective stiffness changes of a microscale RVE caused by the nanoscale uncertainties were investigated. The proposed multiscale modeling method and approach will be a basis for hierarchical multiscale material design of nanocomposite materials.
  • Multiobjective optimization of functionally graded material plates with
           thermo-mechanical loading
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Victor M. Franco Correia, J.F. Aguilar Madeira, Aurélio L. Araújo, Cristóvão M. Mota Soares This work addresses the design optimization of ceramic–metal composite plates with functionally graded material properties, varying through the thickness direction, subjected to thermo-mechanical loadings. Constrained multiobjective optimization is performed for mass minimization and material cost minimization as well as the minimization of stress failure criteria or maximization of natural frequency. The optimization problems are constrained by stress based failure criteria among other structural response constraints and manufacturing limitations. The design variables are the index of the power-law distribution in the metal-ceramic graded material and the thicknesses of the graded material and, eventually, also the metal and ceramic faces.A finite element plate model based on a higher order shear deformation theory, accounting for the transverse shear and transverse normal deformations and considering the temperature dependency of the material properties, is applied for the optimal design of ceramic-metal functionally graded plates. The optimization problems are solved with two direct search derivative-free algorithms: GLODS (Global and Local Optimization using Direct Search) and DMS (Direct MultiSearch). A few multiobjective optimization problems are studied and the results are presented for benchmarking purposes.
  • Experimental investigation of the static behaviour of a corrugated plywood
           sandwich core
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Stephen William Kavermann, Debes Bhattacharyya Sandwich panel structures comprising corrugated plywood core bonded between plywood face sheets were manufactured and tested in flatwise compression and bending. Thin 3-ply Radiata pine veneers were soaked in a hot water bath prior to forming in a heated matched die, resulting in a corrugated profile with a 9 mm height and 43 mm period. Sandwich panel specimens were assembled via a simple process of applying an epoxy adhesive along the joints of the corrugated core and plywood face-sheets and holding in position while the epoxy cured. Through-thickness compressive modulus and strength were tested following the method of ASTM C365. As compared to single layer core, double layer core had a similar modulus, but reduced strength due to instability at the joints between the corrugation layers. Bending behaviour of sandwich beams was investigated in both the corrugated core orientations, revealing that the core was stiffer in shear across the corrugations, but also weaker in this orientation, due to face sheet buckling. This work sets the foundation for future research involving the prediction of properties and experimentation with different core configurations.
  • Focusing on in-service repair to composite laminates of different
           thicknesses via scarf-repaired method
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Wei Feng, Fei Xu, Jialei Yuan, Yuyan Zang, Xiaoyu Zhang In terms of in-service components, sometimes it is difficult to undertake repairing with an autoclave due to the structure configurations and repair conditions. In order to conduct in-service soft patch repair, vacuum bagging process can be used. However, for composite materials manufactured in autoclaves, high porosity levels may be induced when cured using vacuum bagging, which harms the mechanical properties of composites. This article aims to use an alternative material which shows better performance than the original one when using vacuum bagging for the patch. Four groups of scarf repaired composites with various laminate thicknesses were fabricated and tested. Experimental results indicate that the failure strengths of different groups are similar and the dominated failure mode is cohesive failure of adhesive, accompanied by partial 45° and 90° matrix cracks of composite patch. In addition, a finite element model was established to predict the failure strength and explain the damage mechanism. The numerical results show good agreement with test results and indicate that matrix cracks of composites initiate before the adhesive failure. Based on the validated model, the effects of overlap patch and 3D defects on the ultimate strength were discussed.
  • Effect of hygrothermal aging in distilled and saline water on the
           mechanical behaviour of mixed short fibre/woven composites
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Lilla Mansouri, Arezki Djebbar, Samir Khatir, Magd Abdel Wahab In this paper, we investigate the effect of hygrothermal aging in different media on the mechanical behaviour of mixed short fibre/woven composite laminates. We study composite laminates made up of 4 plies of fiberglass fabrics (Mat 300 g/m2, Mat 450 g/m2, Taffetas 800 g/m2) and unsaturated polyester resins, which are realised by contact moulding. In order to properly characterize our materials and to understand the phenomena responsible for modifying their behaviour, various physico-chemical and mechanical tests are carried out on composite laminates with and without a gel coat. The main aim of our study is to determine the mechanical properties, e.g. ultimate stress, yield strength and flexural modulus, of the laminates through three-point bending tests, under the effect of hygrothermal aging in aggressive environments, i.e. distilled water and saline water at different time intervals, and high temperature. In order to follow the physico-chemical evolutions and their influences on the mechanical properties of the composite laminates, a comparative study of the properties is carried out for both unaged and aged specimens. The analysis of all obtained results shows that aging time, medium and temperature have a significant influence on the mechanical behaviour of mixed short fibre/woven composites.
  • Buckling of stomatopod-dactyl-club-inspired functional gradient plates: A
           numerical study
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Peng Liu, Huigao Duan, Le Van Lich, Tuo Ye, Tinh Quoc Bui The translation of natural (or biological) structures into synthetic materials offers a spectrum of feasible pathways towards enhanced or even unprecedented material properties for a wide range of applications. Inspired by excellent mechanical properties of stomatopod dactyl club, herein we propose a new functionally graded material (FGM) model that can adequately describe the characteristics of hierarchical structures in the stomatopod dactyl club. The proposed FGM model is incorporated into extended finite element method with stabilized discrete shear gap to investigate the mechanical buckling behavior of the plate made of materials similar with that in stomatopod dactyl club. The critical buckling factors for cracked FGM plates are computed and analyzed. Numerical results show that the design motif of bioinspired FGM increases the normalized buckling factor, even higher than that of homogenous plates, which suggests that the stability of plates can be enhanced by the use of bioinpsired FGM. The proposed FGM model opens a broad avenue to explore more fascinating mechanical properties of hierarchical gradient materials inspired by stomatopod dactyl club.
  • Modeling of nonlinear response in loading-unloading tests for fibrous
           composites under tension and compression
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Jie Wang, Yi Xiao, Keisuke Inoue, Mashamichi Kawai, Yuande Xue Loading-unloading tests on unidirectional HTS40/PA6 carbon/polyamide laminates are performed in this study to identify the nature of off-axis nonlinear deformation of fibrous composites under tension and compression. Experimental results reveal the involved residual strain (plastic strain) and hysteresis loop during loading and unloading, and their dependence on stress level, fiber orientation, and tension or compression mode. The hysteresis behavior is assumed to be induced by a component of strain, which is considered as anelastic strain. This strain is recoverable like elastic deformation but dissipates work like plastic deformation. An approach is developed to predict the tension-compression asymmetry in plastic strain and anelastic strain, as well as the dependence of asymmetry on stress level and fiber orientation. The proposed approach is a modification of a strength differential model that considers the nonlinear deformation completely as plastic strain for monotonic tests. Predicted off-axis loading-unloading responses of unidirectional laminates are compared with the experimental results. The modified model is found to be effective for adequately describing the tension-compression asymmetry not only in plastic strain but also in complex nonlinear hysteresis behavior.
  • Filler materials in composite out-of-plane joints – A review
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Zsombor Sápi, Richard Butler, Andrew Rhead Out-of-plane joints, such as joints between T, I, Z, L and hat (omega) stiffeners and skin panels are essential part of composite structures. To ensure the integrity of these assemblies, the inclusion of a filler material in the junction region between the skin, flanges and web is necessary. The conventional filler is a rolled unidirectional fibre bundle or resin filling, but there are many other proposed methods in the literature. This paper reviews the state-of-the-art work available in the topic and summarises the proceedings of the past 40 years, in terms of different types of fillers, manufacturing and simulation methods and their effect on joint performance. Possible future areas of interest are additive manufacturing, thermoplastic materials and interleaving, but the biggest challenge is to increase the production rate and manufacturability without reduction in strength and damage tolerance. Moreover, high fidelity finite element analyses and accurate failure prediction methods are needed to exploit the structural role of fillers.
  • An enhanced ply damage model for failure prediction in unidirectional
           composite structures
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): M.W. Joosten The objective of the present study is to develop a validated damage model for simulating progressive damage and failure in unidirectional fibre reinforced composites. In order to represent the evolution of damage two distinct regimes are introduced. The sub-critical regime introduces damage due to the presence of matrix micro-cracks and fibre-matrix de-bonding. Sub-critical damage may result in both damage and plasticity. The post-critical damage model describes strain softening once failure has been detected using the well-established Hashin criteria. A strategy for calculating the required model parameters is presented and two material systems were examined. The present model is capable of predicting the non-linear material response, evolution of sub-critical damage, evolution of irreversible plasticity and ultimate rupture for several laminate configurations. It is envisaged that contemporary analysis approaches, such as the present model, will become widely accepted and allow the full potential of composite designs to be realised.
  • Effects of transverse constraints on the longitudinal compressive strength
           of unidirectional CFRP pultruded plates and rods
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Lichen Wang, Ken'ichi Kawaguchi, Jie Xu, Qinghua Han Unidirectional (UD) carbon fiber reinforced polymer (CFRP) pultruded plates and rods have been widely used for strengthening concrete structures, but their strengthening efficiency has been found to be less under axial compression. Transverse confinement has been shown to improve the longitudinal compressive carrying capacity of UD CFRP composites in practice. However, the lack of a detailed understanding of the relationship between transverse constraints and the improvement of compressive strength leads to a lower material utilization efficiency and a more complicated confinement technique. Therefore, a series of tests and numerical simulations were conducted to analyze and discuss the effect of cross-sectional shape and size, the loading methods, and the transverse constraints on the longitudinal compressive properties of UD CFRP pultruded plates and rods. Some optimized design values of transverse constraints are proposed for the different situations. The results of this investigation can be used to guide the design of the transverse confinement for the hybrid CFRP strengthening technique in concrete columns.
  • Assessment of failure criteria and damage evolution methods for composite
           laminates under low-velocity impact
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Xi Li, Dayou Ma, Huifang Liu, Wei Tan, Xiaojing Gong, Chao Zhang, Yulong Li This study aims to evaluate the applicability of failure criteria and damage evolution methods in the finite element analysis of composite laminates under low-velocity impact. Implemented by the user-defined VUMAT subroutine in ABAQUS, various progressive damage models are used to predict damage initiation and accumulation in a T700GC/M21 composite laminate. Cohesive elements are inserted between adjacent plies to capture interface delamination. The applicability of damage models is investigated by comparing the global mechanical response and distribution of various damage modes. A new variable, equivalent damage volume, is introduced to quantitatively describe the predicted damage when using different models. The numerical results establish that Hashin and Puck failure criteria generate matrix compression damage in more layers of the composite. Maximum stress and Tsai–Wu criteria are not preferred due to their improper predictions in terms of damage area and permanent deformation of the laminate. As for damage evolution laws, the equivalent strain method provides faster stiffness degradation of the laminate and a smaller area of matrix damage compared with the predictions of the equivalent displacement method.
  • Helical bistable composite slit tubes
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Geoffrey Knott, Andrew Viquerat Bistability in doubly curved and twisted (helical) composite slit tubes is investigated for the first time. This work establishes a natural extension in this area which has been focused on straight and until more recently, doubly curved (toroidal) tubes with positive Gaussian curvature. The model developed introduces longitudinal and transverse curvature, and twist into strips of laminated composite material. The composite is engineered to be bistable and the second stable state determined via strain energy minimisation using the Rayleigh-Ritz method. The strain energy is formulated as a function of curvature strains, longitudinal stretching and a variable middle ply fibre angle of the laminate. The second stable state forms a compact and untwisted cylindrical coil with the latter engineered by tailoring the middle ply fibre angle. A new manufacturing process capable of producing helically curved tubes using glass-fibre/polypropylene-matrix composite is presented to verify the hypothesis of this work. An untwisted coil enables the efficient stowage and deployment of new forms of bistable composite tube which adhere to similar form factors as straight and toroidal ones. By embedding electrical conductors, helical bistable composites enable new lightweight, compact and multifunctional structures for communication and sensing applications.
  • Guided wave propagation in a multilayered magneto-electro-elastic curved
           panel by Chebyshev spectral elements method
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Dongliang Xiao, Qiang Han, Tengjiao Jiang This paper investigates the dispersion characteristics of guided waves in a multilayered magneto-electro-elastic curved panel. Based on the Hamilton principle, the Chebyshev spectral element method is applied along the radial and circumferential directions to obtain the dispersion equation of the waves. The advantage of the spectral element method over a high degree piecewise polynomial basis function is the high order of accuracy. The influences on the dispersion characteristics is analyzed in terms of four factors: structural radian, magneto-electro effect, thickness-diameter ratio, and stacking sequence. In addition, the dispersion curves in the curved panel display a phenomenon which different from plate and hollow cylinders: mode conversion. We explain this from the perspective of physics and mathematics, separately. The structural radian and thickness-diameter ratio show the most significant effect on the mode conversion.
  • Uncertainty analysis of mechanical properties of plain woven carbon fiber
           reinforced composite via stochastic constitutive modeling
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Chao Zhu, Ping Zhu, Zhao Liu Carbon fiber reinforced polymer (CFRP) composites possess inevitable geometric variabilities across scales, which directly relates to the fluctuations of mechanical properties. Meanwhile, complex damage and failure are involved in material behaviors. In this study, a phenomenological damage constitutive law of plain woven CFRP was first established based on experimental data. In order to further characterize the uncertainty of mechanical properties, a stochastic constitutive model was developed. A series of statistical volume element (SVE) finite element models were constructed according to the geometrical parameters extracted from micro-computed tomography (micro-CT) image. A set of mechanical response data including damage and failure stages were obtained from SVE simulation results. Uncertainty was introduced into several constitutive parameters and was then quantified by statistical Vine Copula method, margin characteristics, as well as correlations among these parameters, were analyzed. The proposed stochastic constitutive model was verified through comparison with experimental results, which showed the ability of the model to express the randomness of mechanical properties.
  • Dynamic mechanical behavior of flocked layer composite materials
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Karoly F. Fodor, Vijaya Chalivendra, Yong K. Kim, Armand F. Lewis A custom designed dynamic mechanical apparatus was constructed to measure the tan(δ) (loss tangent) properties of Flocked (layer) Energy Absorbing Materials (FEAM). Flocked Impact Energy Absorbing (IEA) materials derive their dynamic mechanical impact absorbing properties from the compressional deformation (bending, buckling) and inter-fiber friction of nominally upright flocked fibers in the flock fiber composite assembly. The reported work summarizes the dynamic mechanical, low strain, linear viscoelastic tan(δ) properties of FEAM composite layer structures at frequencies from 25 to 100 Hz and a temperature range of 0 °C–50 °C. The effect of tan(δ) at various flock densities (fibers/mm2) was also measured. These studies were carried out to gain information and insight into the role that a material’s dynamic mechanical properties have on the overall IEA performance of sport (and military) apparel pad structures. Results show that the tan(δ) of FEAM materials increases as the measuring frequency and flocked layer’s flock density increase. Tan(δ)’s behavior at various temperatures (0 °C–50 °C) did not follow an expected trend; the highest tan(δ) value was observed to occur at room temperature. Nevertheless, the obtained tan(δ) information on these various FEAM configurations should provide some guidance in designing optimized FEAM sport (and military) pad IEA composites.
  • Vibration analysis of deploying laminated beams with generalized boundary
           conditions in hygrothermal environment
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): L. Wang, M. Xu, Y.H. Li The free vibration of a deploying laminated beam in hygrothermal environment with a constant axial velocity is studied. The model of this system is given within the framework of the Euler-Bernoulli beam theory and von Karman nonlinear strain theory. The nonlinear dynamic equilibrium equation with generalized boundary conditions is established based on the Hamilton’s principle with considering the combined effects of the axial motion, transverse vibration and hygrothermal environment. Based on the Galerkin method, a set of ordinary differential equations is obtained. The numerical results of the discretization equation are performed adopting the eigenvalue method and Newmark method. In addition, the dynamic stability is discussed, and extensive numerical calculations are performed to illustrate the effects of varying extension velocities, temperature, humidity and ply angles on frequencies.
  • CFRP manufacturing method by using electro-activated deposition and the
           effect of reinforcement with carbon fiber circumferentially around the
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Kazuaki Katagiri, Shinya Honda, Sayaka Minami, Yusuke Tomizawa, Daiki Kimu, Shimpei Yamaguchi, Takuya Ehiro, Tomoatsu Ozaki, Hirosuke Sonomura, Sonomi Kawakita, Mamoru Takemura, Yayoi Yoshioka, Katsuhiko Sasaki For the efficient manufacturing of carbon fiber reinforced plastics (CFRP), the electro-activated deposition resin molding (ERM) method is developed. The carbon fiber fabric is immersed in an electro-activated deposition solution containing polymer with epoxy groups. By energization, an epoxy resin is precipitated on the surface of the carbon fiber, and impregnation occurred in the solution. In this study, applying the ERM method, CFRP specimen with the hole was manufactured. Furthermore, the reinforcement by arranging the fiber circumferentially around the hole was installed. The carbon fiber fabric was sewn using the tailored fiber placement (TFP) machine. As a results, the effect of the reinforcement was confirmed. The tensile strength of CFRP with the hole and the reinforcement was the same as in the case without the hole. From the finite element analysis, it was confirmed that the reinforcement around the hole reduced the stress concentration.
  • The development and ballistic performance of protective steel-concrete
           composite barriers against hypervelocity impacts by explosively formed
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Alex Remennikov, Edward C.J. Gan, Tuan Ngo, Michael D. Netherton Explosively formed projectiles (EFP) are one of the most severe explosive and impact loading threats for civil infrastructure and military vehicles. Currently, there is no effective means of protection for military vehicles and infrastructure facilities from EFPs. This paper presents the experimental results of the hypervelocity impact of EFPs on steel-concrete (SC) barrier systems of finite dimensions. The SC barrier units tested were broadly representative of the type of protective SC units used in the expedient construction of barriers for mitigating improvised explosive device (IED) and EFP threats to critical infrastructure facilities. The response of non-composite, partially-composite, and fully-composite SC barrier units was studied. All studied protective systems were capable of terminating the high-velocity projectiles effectively through the combined action of the concrete core and steel faceplates. The data gathered from these tests are also intended to further the understanding of impacts on SC composite structures at speeds greater than 1000 m/s and for the calibration of numerical models of EFP interaction with SC targets. 3D numerical simulations were performed to better understand the various stages of EFP interaction with the SC composite barriers and develop recommendations for their design optimisation. No previously published results on the EFP terminal ballistic performance of SC composite structures of finite dimensions have been found in the open literature.
  • A parametric study of flutter behavior of a composite wind turbine blade
           with bend-twist coupling
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Praveen Shakya, Mohammed Rabius Sunny, Dipak Kumar Maiti Now a days wind turbine blades are generally designed to have length as high as 60 m or more to maximize power production. Aeroelastic instabilities such as flutter are major concerns for these long, flexible and slender blades. Stiffness coupling between bending and twisting modes can be used to improve the aeroelastic performance of such blades. In composite blades bend twist coupling can be achieved by imparting unbalance in the lamination sequence. In the present work, a parametric study has been conducted to study the effect of unbalances in different parts of a wind turbine blade on flutter instability. An eigenvalue-based approach has been used for flutter analysis. It has been observed that flap-torsional stiffness has high impact on critical flutter speed. The critical flutter speed is increased by 40% with flap-torsional stiffness due to the unbalance in the entire section of the blade with symmetric skin, while for asymmetric skin; the achievable increment is 100%. The unbalance in the spar cap of the blade has less critical flutter speed as compared to the unbalance in the entire section of the blade. Unbalance in the entire section of the blade with asymmetric skin can lead to highest flutter speed.
  • Hot spot analysis in complex composite material structures
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Henrik Molker, Renaud Gutkin, Silvestre Pinho, Leif E. Asp In this paper, failure initiation in composite structures due to high out-of-plane load components is predicted. The predictions are based on finite element models built with shell elements, intended for global models. The full 3D stress state is estimated through stress recovery by the extended 2D FEM approach. Failure initiation is predicted with state of the art failure criteria for transversely isotropic composite materials. The approach is validated for a range of geometries with different modelling resolutions. Finally, the methodology is verified on a complex composite structure. With the proposed approach, using shell elements, efficient modelling strategies of large structures can be pursued using hot spot analyses to identify critical locations.
  • Guided waves propagation in anisotropic hollow cylinders by Legendre
           polynomial solution based on state-vector formalism
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Mingfang Zheng, Cunfu He, Yan Lyu, Bin Wu A spectral approach was presented in the computation of dispersion curves for the general anisotropic hollow cylinders. The derivation is based on the hybrid method of the state-vector formalism and Legendre polynomials expansion, which was previously adopted for the anisotropic plates. This method will lead to an eigenvalue/eigenvector problem for the calculation of wavenumbers and displacement profiles. This hybrid method avoids solving the transcendental dispersion equation. A closed-form solution for the hollow cylinder, involving multiple integral expressions, is demonstrated. A stable scheme for the integration expansion was established by re-expanding the expansion operators from the first round Legendre polynomial expansion versus the displacements. Usually, the traditional matrix methods are based on root-finding algorithms, which is difficult to implement in anisotropic tubes. In this research, the hybrid approach we proposed provides a reliable mathematical solution of wave propagations in an anisotropic hollow cylinder. Applications will be illustrated using isotropic and orthotropic hollow cylinders, in which the isotropic case agrees well with the results by global matrix method. The dispersion curves of orthotropic hollow cylinders, when the out radius set to approximate infinity, are compared to its corresponding anisotropic plate, which is obtained from our previous work. Furthermore, the displacement and stress profiles will be given and analyzed for an orthotropic tube, which has 10 mm thickness with an out radius of 50 mm.
  • Mechanical, morphological, structural and dynamic mechanical properties of
           alkali treated Ensete stem fibers reinforced unsaturated polyester
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Tolera A. Negawo, Yusuf Polat, Feyza N. Buyuknalcaci, Ali Kilic, N. Saba, M. Jawaid Present study deals the surface morphology and structural composition analysis of alkali (NaOH) treated 2.5% 5.0% and 7.5 wt% Ensete stem fiber obtained from the Ethiopian Ensete ventricosum plant. Treated Ensete fibers reinforced unsaturated polyester (UP) composites were characterized in terms of tensile, flexural, surface morphology and dynamic mechanical properties. Mechanical test results revealed that 5.0 wt% alkali treated Ensete fibers/UP composites showed 14.5% and 43.5% increase in flexural strength and Young's modulus respectively, with relative to untreated Ensete fibers/UP composites. Storage and loss modulus value also highest for 5.0 wt% alkali treated Ensete fibers/UP composites. Moreover, a positive shift in glass transition temperature (Tg) of composites after alkali treatment and tensile fracture surface morphology indicates better interfacial interaction in treated Ensete fibers/UP composites. Overall we concluded that 5.0 wt% treated Ensete fibers satisfactorily and effectively improved mechanical, morphological and dynamic properties of UP for various engineered and hi-tech applications.
  • Electrochemical performance of corroded reinforced concrete columns
           strengthened with fiber reinforced polymer
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Hongjun Liang, Shan Li, Yiyan Lu, Jiyue Hu, Zhenzhen Liu External bonding of fiber reinforced polymers (FRP) has been widely used to strengthen corroded reinforced concrete columns. During the application of FRP, the variations of electrochemical parameters, such as corrosion current density and charge transfer resistance, have a significant effect on the understanding of anti-corrosion protection mechanism of FRP. Therefore, this study conducted the electrochemical measurements of corroded reinforced concrete wrapped with FRP. The considered variables included pre-corrosion degrees (1%, 3%, and 6% theoretical mass losses) and anti-corrosion protection methods (non-protection, carbon FRP wrapping, glass FRP wrapping, and epoxy coating). The measurements included half-cell potential, linear polarization, and electrochemical impedance spectroscopy (EIS). By analyzing the variation of the above electrochemical parameters in the whole exposure period, it was concluded that FRP wrapping cannot stop the elicited corrosion response but were effective to reduce the corrosion rates, and the main contributor was the epoxy used as an adhesive between concrete and FRP. Moreover, clear inductive loops were observed in Nyquist plots. According to the fitting results, the inductance values were significantly larger for FRP wrapped specimens and less corroded specimens. The inductance may be owing to the difficulties for diffusion of corrosion products.
  • In-plane mechanics of a novel cellular structure for multiple morphing
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Weidong Liu, Honglin Li, Jiong Zhang, Yalei Bai Cellular structures are potential candidates as underlying supports for flexible skins due to their outstanding characteristics such as light weight, zero Poisson’s ratio and low effective moduli. This paper develops a novel zero Poisson’s ratio cellular structure composed of semi-periodic sinusoidal beams with light weight, low in-plane moduli and high strain capability. The equivalent in-plane elastic and shear moduli of the structure are studied through a combination of theoretical and finite element analysis. Then the maximum global strains are investigated to show the morphing capability of the structure. Results show that the in-plane moduli of the cellular structures are several orders of magnitude lower than the raw material. The global strains of the cellular structures could be over 10 times greater than the raw material. The results also demonstrate the potential of the proposed structure for uniaxial, shear or hybrid in-plane morphing.
  • Straight FRP anchors exhibiting fiber rupture failure mode
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Enrique del Rey Castillo, Michael Griffith, Jason Ingham The purpose of the work was to characterize the behavior of FRP anchors installed at the end of FRP sheets and develop a methodology to calculate the capacity when the anchors exhibited the fiber rupture failure mode. To do so an extensive experimental program was undertaken and the force transfer mechanism between the anchor and the structure was studied in depth. A theoretical model and then an adapted simplified model were developed and empirically calibrated with the experimental results, which yielded a number of design equations. The main observation from the experimental results is that the anchor was weaker when the fanning angle increased and the efficiency dropped as the anchor increased in size. The final contribution to the knowledge is a method to calculate the capacity of anchors installed at the end of FRP sheets when exhibiting fiber rupture failure mode.
  • Real-time electrical impedance monitoring of carbon fiber-reinforced
           polymer laminates undergoing quasi-static indentation
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Khaled Almuhammadi, Arief Yudhanto, Gilles Lubineau Laminated composites are vulnerable to damage from out-of-plane loading, particularly impact loading, and the incurred damage is often only detected by evaluating the post-impact condition of the composites. Real-time monitoring techniques are desirable for early detection of damage. Utilizing changes in the electrical properties of composites to track incurred damage is promising, but the interpretation of such measurements is still challenging. Here, an electro-mechanical system is introduced to understand how well we could detect mechanical degradation in carbon-fiber-reinforced polymer (CFRP) plates undergoing a quasi-static indentation (QSI) test, which is representative of an impact load. The system measures the in situ, real-time changes in impedance and phase angle along the specified conductivity paths. Two different electrode configurations are proposed and tested. In all studied cases, the system effectively detected severe damage, characterized by an immediate reduction in strength, in CFRP. Using our proposed electrode configurations, we discovered that the early detection of barely visible damage strongly depends on two factors: (i) the location of the injection-measurement points with respect to the damage, and (ii) the orientation of the measurement paths with respect to the fibers orientation in the laminated CFRP surface.
  • A modified Fourier solution for sound-vibration analysis for composite
           laminated thin sector plate-cavity coupled system
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Hong Zhang, Dongyan Shi, Shuai Zha, Qingshan Wang This paper applied the modified Fourier series method to investigate the sound-vibration characteristics by establishing a composite laminated thin sector plate-cavity coupled model for the first time based on the classical plate theory (CPT) and Rayleigh-Ritz energy technique. The coupled system consists of an annular sector or circular sector plate backed by an acoustic cavity filled with air or water. Ignoring the influence of boundary conditions, displacements admissible functions of laminated sector plate and sound pressure admissible functions of cavity can be set up as a Fourier series superposition, whose composition are the superposition of Fourier cosine series and supplementary functions. The addition of these supplementary polynomials can effectively eliminate the discontinuity or jump phenomenon on the boundary. The correctness of the established analytical model has been validated by being compared with the results achieved by the finite element method (FEM). On this basis, the coupling mechanism of the weakly coupled system and the strongly coupled system are discussed in detail. In addition, some new results and discussions are given, including the cavity depth, plate thickness, anisotropic degree, varying boundary conditions and so on, which could provide reference for future research.
  • Fatigue crack growth characterization in adhesive CFRP joints
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): I. Floros, K. Tserpes Adhesive joints find an increasing use in lightweight structures, which is proportional to the evolution of carbon fiber-reinforced plastics (CFRPs). Understanding of fatigue crack growth behavior in adhesive CFRP joints is essential for the efficient maintenance and repair of existing joints and the design of new joints. Here, the fatigue crack growth behavior of adhesive CFRP joints under Mode-I, Mode-II and Mixed-Mode I + II loading conditions is characterized experimentally by means of Mode-I fatigue fracture toughness tests, Mode-II fatigue fracture toughness tests and the Mixed-Mode fatigue lap shear test. For the three different tests, the Double Cantilever Beam (DCB), the End-Notch Flexure (ENF) and the Cracked Lap Shear (CLS) specimens are used, respectively. Crack growth versus number of cycles is reported and modified Paris-laws are derived. The DCB specimens failed in cohesive failure mode while the ENF and CLS specimens in adhesive. The crack growth in the DCB specimens was more stable and showed a smaller scatter among the different specimens than the ENF specimens. Crack propagation with number of cycles in CLS specimen was almost linear. The results reported herein suggest a full experimental characterization of fatigue crack growth behavior of the considered aerospace CFRP/adhesive material system and can be proved very useful in the development and validation of fatigue crack growth simulation models.
  • Three-dimensional nonlinear bending analysis of FG-CNTs reinforced
           composite plates using the element-free Galerkin method based on the S-R
           decomposition theorem
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Tao Zhou, Yanqi Song A three-dimensional element-free Galerkin method (EFG) based on the Strain-Rotation (S-R) decomposition theorem, named 3D-SR-EFG, is developed to investigate the nonlinear bending behavior of functionally graded carbon nanotube reinforced composite (FG-CNTRC) plates. Due to its overcoming the deficiencies of classic finite deformation theories, S-R decomposition theorem can provide a reliable theoretical support for the geometrically nonlinear simulation. The incremental variational formulation based on the S-R decomposition theorem for three-dimensional static large deformation problems is derived from the updated co-moving coordinate formulation and principle of potential energy rate. Global weak-form EFG is adopted to obtain the discrete form of the formulation. Convergence and comparison studies are conducted to validate the numerical stability and accuracy of the proposed 3D-SR-EFG. The influences of volume fraction and distributions of CNTs, plate’s aspect ratio and width-to-thickness ratio, boundary conditions on the nonlinear bending response of the CNTRC plates are numerically analyzed and discussed in parametric studies. Results demonstrate that the 3D-SR-EFG can effectively predict the nonlinear bending behavior of the CNTRC plates. This work also further extends the applications of the S-R decomposition theorem.
  • On a multi-scale finite element model for evaluating ballistic performance
           of multi-ply woven fabrics
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Emre Palta, Howie Fang In this study, an improved multi-scale finite element model of woven fabric was developed for evaluating ballistic responses of multi-ply woven fabrics. The improved multi-scale model was composed of a meso- and a macro-scale model that were coupled through node-sharing at the interface to improve wave propagation responses. To evaluate the accuracy and efficiency of the multi-scale model, a full meso-scale model was also created, and both numerical models were validated using experimental data. The multi-scale model was found to have good accuracy and computational efficiency compared to the full meso-scale model and thus used in the evaluation of ballistic responses of multi-ply woven fabrics. Further, multi-ply woven fabrics were also created using the multi-scale model, and they were validated against experimental data. Thereafter model validations, the three-, five-, seven-, and ten-ply fabrics were created using the improved multi-scale model to investigate ballistic performance of the multi-ply dry Kevlar woven fabrics. The ballistic limit velocities of each target were first determined, followed by the ballistic impact responses, energy transitions, displacements, and damage patterns of the four multi-ply at the impact of V100 ballistic limit were discussed in detail.
  • Numerical failure analysis of built-up columns composed of closely spaced
           pultruded FRP channels
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Fabio Minghini, Nerio Tullini, Francesco Ascione, Luciano Feo The results of geometrically nonlinear analyses on 43 built-up Pultruded Fibre-Reinforced Polymer (PFRP) columns with closely spaced chords and intermittent interconnections are presented. A comparison between columns with the end sections entirely loaded and columns loaded at the end battens only is reported, showing no appreciable difference in the P-δ response. The effects due to variations of column length and battens spacing are then investigated. It is found that stocky columns with small battens spacing attain pre-buckling failure at the web-flange junctions of the chords for loads approximately equal to 70% of the crushing load. Slender columns fail by global buckling, whereas intermediate-slenderness columns may experience interaction between local and global buckling. A design method is finally proposed.
  • Mechanical characterization of CFRP/steel bond cured and tested at
           elevated temperature
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): E.R.K. Chandrathilaka, J.C.P.H. Gamage, S. Fawzia Glass transition temperature (Tg) of the bond between CFRP and steel influences on the service and fire performance of strengthened members. A total of eighty-two CFRP/steel double strap joints were prepared and tested under elevated temperature. They were cured under a range of elevated temperature conditions in the control laboratory environment and in the open environment which is practically feasible in large civil engineering structures. The test results showed a similar trend of reductions in the bond strength, Poisson’s ratio and Elastic modulus of CFRP/steel joint with the exposure to the elevated temperature. More than 50% reduction in the Poisson’s ratio, elastic modulus and the bond strength was noted when the bond line temperature exceeds Tg + 15 °C, irrespective of the curing time and curing conditions. Initial elevated temperature curing also causes for shifting the curves in the right-skewed direction. A significant increase in Tg of bond was noted with 4 h initial curing at 75 °C, i.e. Tg + 20 °C.
  • Minimum torsional reinforcement ratio for reinforced concrete members with
           steel fibers
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Hyunjin Ju, Deuck Hang Lee, Kang Su Kim The current design codes for concrete structures suggest the minimum torsional reinforcement ratio to induce minimum ductile failure after cracking in reinforced concrete (RC) members subjected to torsional moment. However, as the member strengths are quite often lower than the torsional cracking moment strengths even when satisfying minimum torsional reinforcement ratio specified in the current codes, it may not serve the original purpose of preventing brittle failure immediately after cracking. In this study, a rational equation is thus presented for calculating the minimum torsional reinforcement ratio that could provide a sufficient margin of safety in design. The minimum fiber factor to be applied in steel fiber reinforced concrete (SFRC) members is also proposed. The proposed model was found to reflect the effects of torsional cracking strength, longitudinal and transverse reinforcement ratio, and the presence of torsional reinforcement in concrete members. It was also found that as the reserved strength included in the proposed equation is determined based on the test results of the RC members, it contains an appropriate factor of safety for the RC members. In addition, it was confirmed that the minimum fiber factor is conservative compared with the collected test results of the SFRC specimens.
  • A new aero-structural optimization method for wind turbine blades used in
           low wind speed areas
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Han Yang, Jin Chen, Xiaoping Pang, Gang Chen The wind turbine blades used in lower quality wind resources have been recently designed. The designed blades for low wind speed areas are scaled version of blades used in high wind speed sites with longer blade lengths, same chord length and same airfoils series. A discrete optimization method for wind turbine blade used in low wind speed sites is proposed. In the optimization process, both aerodynamic and structural parameters are considered as design variables. The airfoil design is also considered in the optimization process to improve the aerodynamic and structural performance by the method of airfoil integrated theory. The Blade Element Momentum theory is used to evaluate the blade aerodynamic performance and the Classical Laminate Theory is used to estimate the stiffness and mass per unit of each blade section. Finally, a Finite Element Method structural analysis is used to verify the strength under the extreme loading conditions. To prove the efficiency and robustness of the design, a commercial 2.1 megawatt HAWT blade is used as a case study. The results show that the aerodynamic and structural performance of the new blade is improved compared with that of the original one.
  • Optimized design for projectile impact survivability of a carbon fiber
           composite drive shaft
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): T.C. Henry, B.T. Mills Helicopter power transmission systems consist of aluminum drive shafts which are connected together with flexible couplers to accommodate misalignment and hanger bearings to secure each drive shaft to the airframe. The development of carbon fiber composites have resulted in designs which reduce the weight of the system as well as eliminate parts which can be integrated into the composite material itself. The reduction in weight and parts provide transmissions which are more maintenance cost conscious as downtime related to manual part inspection for defects and damage is reduced. One barrier to fielding composite systems is impact qualification to ensure passing of survival standards. Some certainty that a design will not increase the vulnerability of a vehicle must exist and this paper provides an impact study for flexible composite drive shafts and performs a design analysis on tradeoffs between weight, and residual strength. 16 drive shafts with two different thicknesses and the optimized lamination [±456/±402] were manufactured and impacted with either 7.62 or 12.7 mm projectiles while under 252 N-m of torque.
  • In-plane crushing behaviors of piecewise linear graded honeycombs
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Zhen Li, Yi Jiang, Tao Wang, Liangmo Wang, Weichao Zhuang, Dan Liu The cell-wall thickness induced and cell-wall angle induced piecewise linear graded honeycombs (PLGHs) are presented and investigated under in-plane compression. Effects of cell-wall angle on the strength of honeycombs are discussed. Results reveal that the normalized crushing strength displays similar variety to that of the shape ratio within a given range. Based on this, traditional theoretical models of plateau stresses are improved. Comparative studies on different kinds of PLGHs are conducted. Quasi-static and dynamic deformation mode, strength and energy absorption capacity are intensively discussed. Both weak-to-strong and top-to-bottom layer-by-layer deformation mode are observed. Comparison of strain-stress curves shows that cell-wall angle induced PLGHs generally express lower level of stress than that of cell-wall thickness induced PLGHs under quasi-static compression, but the difference is insignificant as the crushing velocity increases. Theoretical prediction is also conducted to estimate the stress level under quasi-static and high crushing velocities. Both the constant velocity and initial velocity loadings are applied to studying the energy absorption capacity. It is interesting to note that cell-wall angle induced PLGHs shows a time saving characteristic as the normalized initial kinetic energy increases from 0.75 to 1, which has not been discovered in other types of honeycombs.
  • Fully coupled thermomechanical analysis of laminated composites by using
           ordinary state based peridynamic theory
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Yan Gao, Selda Oterkus This study presents fully coupled ordinary state based peridynamic (PD) model for of laminated composites. The formulation includes coupling of both thermal and mechanical fields. In order to verify the proposed model, numerical simulations for benchmark problems are carried out and their results are compared with the ones from ANSYS solutions. First, the thermomechanical behaviour of the laminated composites subjected to both uniform and linear temperature changes are tested for single and multi-layer composites. Then, fully coupled thermo-mechanical formulations are validated for laminated composites subjected to pressure shock. Finally, the crack propagation paths and temperature distributions are predicted for shock loading conditions. In conclusion, the present PD model is well suited for solving fully coupled thermomechanical problems for laminated composites including crack initiations and propagations.
  • Comparison of two progressive damage models for studying the notched
           behavior of composite laminates under tension
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Johannes Reiner, Thomas Feser, Dominik Schueler, Matthias Waimer, Reza Vaziri Two continuum damage models with different underlying assumptions are investigated to assess their predictive capabilities and limitations with respect to progressive intra-laminar damage in notched IM7/8552 CFRP composite laminates under tension.The recently modified continuum damage model, CODAM2, implemented in LS-DYNA is compared to a Ladevèze-based damage model, ABQ_DLR_UD, implemented as a user-material model (VUMAT) in Abaqus/Explicit. Fundamental similarities and differences between the two models are first investigated by various single element simulation case studies before assessing their predictive capabilities at the more global coupon level. Over-height Compact Tension (OCT) tests with dispersed and blocked lay-ups as well as a wide range of scaled Center-Notched Tension (CNT) specimens are used to evaluate characteristic damage-related quantities such as strength, post-peak behavior and fracture energies. Experimental data are also used to further validate the two different meso-scopic damage models.The results of this study clearly demonstrate the layups, geometries and loading conditions that are suitable for applying intra-laminar damage models with confidence to this class of CFRP material system. Conversely, the study shows limitations of the continuum damage modeling techniques and suggests strategies that can be used to address these drawbacks.
  • An Octet-Truss Engineered Concrete (OTEC) for lightweight structures
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Parham Aghdasi, Ian D. Williams, Brian Salazar, Nicole Panditi, Hayden K. Taylor, Claudia P. Ostertag Recent advances in the development of Ultra-High Performance Fiber-Reinforced Concrete (UHP-FRC) with very high compressive strength has inspired the development of a lightweight structure by engineering the void spaces in the material, thus taking advantage of porous concrete’s thermal insulating properties while maintaining strength and stiffness. This paper refers to this engineered material as Octet-Truss Engineered Concrete (OTEC). To make OTEC structures, UHP-FRC and “green” UHP-FRC (G-UHP-FRC) mixtures were developed. 50.8-mm side-length OTEC unit cell specimens with various element diameters as well as 5 × 1 × 1-cell OTEC flexural specimens with 8 mm-diameter elements were cast and tested under uniaxial compression and four-point bending, respectively. The compressive strength of the OTEC unit cell specimens with various element diameters is mainly stretching-dominated, and hence considerably surpasses that of the control foam Green Ultra-High Performance Concrete specimens with random pore orientations. These results indicate a promising application of UHP-FRC and G-UHP-FRC OTECs for lightweight structures.
  • Numerical research on elasto-plastic behaviors of fiber-reinforced polymer
           based composite laminates
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Rui Ren, Guigao Le, Jianlin Zhong, Dawei Ma, Qiang He A new pressure-dependent elasto-plastic constitutive model for polymer based composite laminates with unidirectional plies is established and implemented into finite element software to simulate off-axis loading tests of two kinds of composites. The validity of the new plasticity model is proved by available off-axis loading test results. Additionally, accuracies and efficiencies of the backward Euler algorithm and the forward Euler algorithm respective in solving plastic deformation of fiber-reinforced composite materials are analyzed. Compared with the forward Euler algorithm based plasticity model, the backward Euler scheme shows comparative prediction accuracy of laminate plastic behavior and better computational efficiency. This paper is expected to facilitate the numerical predictions of composite laminates’ elasto-plastic behaviors for researchers and engineers.
  • Repair of severely-damaged RC exterior beam-column joints with FRP and
           FRCM composites
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Flora Faleschini, Jaime Gonzalez-Libreros, Mariano Angelo Zanini, Lorenzo Hofer, Lesley Sneed, Carlo Pellegrino This paper presents the results of an experimental program aimed at comparing the cyclic behavior of three full-scale reinforced concrete (RC) exterior beam-column joints repaired with externally bonded composites. Preliminary, the specimens suffered significant damage, which resulted in a beam-joint (B + J) failure, i.e. shear failure of the panel joint after yielding of longitudinal reinforcement in the beam. The damaged specimens were then repaired using either Fiber Reinforced Polymer (FRP) or Fiber Reinforced Cementitious Matrix (FRCM) composites, and the same loading history was applied to the repaired specimens. The experimental behavior was compared with that of original specimens, and results are presented in terms of load-carrying capacity, stiffness deterioration, ductility, dissipated energy, and equivalent damping viscous ratio. Results allow to clearly identify the contribution of the externally-bonded composites to the overall behavior of the repaired specimens.
  • Nonlinear vibration of piezoelectric sandwich nanoplates with functionally
           graded porous core with consideration of flexoelectric effect
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): S. Zeng, B.L. Wang, K.F. Wang The porous materials become a new class of advanced engineering material due to their excellent advantage such as low specific weight, efficient capacity of energy dissipation, reduced thermal and electrical conductivity, enhanced recyclability and machinability. In this study, the nonlinear vibration of piezoelectric sandwich nanoplates with functionally graded (FG) porous core under electrical load is presented. The piezoelectric effect, flexoelectric effect and von Karman type large deformation are simultaneously taken into account. Results show that the piezoelectric and flexoelectric effects of the material reduce the vibration frequency of the sandwich nanoplate even there is no applied voltage on the piezoelectric layer. The natural frequency of the sandwich structure with porous core can be adjusted by controlling the porosity distribution and porous coefficients of the porous material, and the applied voltages on the piezoelectric layer.
  • Kink band predictions in fiber composites using periodic boundary
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): A.B. Jensen, J. Thesbjerg, J.L. Wind, H.M. Jensen A finite element based scheme for modelling kink band formation in fiber composites was developed. The model is computationally efficient by requiring only a few number of fiber/matrix layers representing the microstructure of the composite material. Yet, in comparison to finite element models including many fiber/matrix layers, the model is accurate by specifying periodic boundary conditions along the sides of the representative volume elements. These periodic boundary conditions allow for a rotation of the kink band after its initiation as observed in full-scale finite element calculations and experiments. Three different computational schemes for determining the kink band rotation have been suggested and compared.
  • Dynamic indentation of auxetic and non-auxetic honeycombs under large
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): L.L. Hu, M.Zh. Zhou, H. Deng It is generally acknowledged that the indentation resistance or hardness of auxetic materials is higher than that of their conventional counterparts under elastic deformation. However, this property of the auxetic material may not always be superior to that of the non-auxetic materials when the deformation is relatively large with plasticity considered. In this study, we come up with an index to quantitatively depict the indentation resistance of the hexagonal honeycombs under large deformation. The indentation resistance of both the auxetic and non-auxetic hexagonal honeycombs is compared and discussed. Results show that in the premise of honeycombs possessing the same relative density, the indentation resistance of auxetic hexagonal honeycombs is not always higher than that of the non-auxetic honeycombs. This phenomenon is verified by the numerical simulations. Further analysis shows that there is a critical value of the absolute value of Poisson’s ratio, which is determined by the cell-wall length ratio, to estimate the higher indentation resistance between the auxetic and non-auxetic hexagonal honeycombs. The influence of indentation velocity is also analyzed based on numerical simulations. This present work is supposed to shed light on the design and evaluation of the indentation resistance for both auxetic and conventional honeycombs.
  • Three-dimensional vibration analysis of beams with axial functionally
           graded materials and variable thickness
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Mingfei Chen, Guoyong Jin, Yantao Zhang, Fenglei Niu, Zhigang Liu In this paper, the vibration problem of beams with axial functionally graded materials (FGMs) and variable thickness is firstly investigated by isogeometric analysis (IGA) in conjunction with three-dimensional (3D) theory. Based on guaranteeing geometry exactness of the strength of non-uniform rational B-splines (NURBS), the curves of non-uniform thicknesses of beams are exactly described. Two beams models (slender model and plump model) are taken into account. Then the material properties smoothly varying in axial direction are calculated by two types of material distributions, power-law and exponential law. The requirement for the weak form of FGMs beams is easily satisfied as NURBS can provide higher order derivative. In numerical results, the convergence is demonstrated, then the accuracy of the current work is validated through comparing solutions with those from the commercial package ANSYS. Moreover, the effects of geometrical proprieties, material parameters as well as boundary conditions on the frequency are also examined.
  • An accurate and straightforward approach to thermo-electro-mechanical
           vibration of piezoelectric fiber-reinforced composite cylindrical shells
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Zhenhuan Zhou, Yiwen Ni, Shengbo Zhu, Zhenzhen Tong, Jiabin Sun, Xinsheng Xu Exact solutions for thermo-electro-mechanical free vibration of a piezoelectric fiber-reinforced composite (PFRC) cylindrical shell is obtained under the framework of Hamiltonian mechanics. Two types of fiber reinforcement including the uniformly distributed in each ply and functionally graded in the thickness direction are taken into considered. Unlike the classical analytical treatment which relies on trial functions, the governing equations in the Hamiltonian form can be directly solved through a rigorous way. Free vibration of PFRC cylindrical shells is reduced into a symplectic eigenproblem which has five kinds of explicit eigenfunctions. Analytical frequency equations and vibration mode shapes are derived simultaneously. In numerical examples, a comparison study is performed to verify the validity of the proposed solution. A detailed discussion is presented to reveal the effects of key influencing factors on the expression of eigenfunctions. Some new results which can be used as benchmarks for the approximate approach are given also.Graphical abstractGraphical abstract for this article
  • Dynamics of a functionally graded Timoshenko beam considering new
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Ufuk Gul, Metin Aydogdu, Fatih Karacam Dynamics of a functionally graded (FG) beam were studied using Timoshenko and Euler-Bernoulli (EB) beam theories. Wave propagation of infinite beams and vibration analysis of simply supported FG beams were investigated. Variation of the material properties were assumed in the power law form. It was obtained that unlike isotropic beams, axial and transverse waves are coupled in FG beams due to unsymmetric material variation in the thickness direction of the beam. Two and three dispersion curves were obtained for EB and Timoshenko beam theory, respectively. All of these modes are axial-flexural coupled modes and coupling degree depends on material distribution with respect to mid-surface of the beam. The spectrums of different beams may be classified considering corresponding mode shapes. Wave propagation and vibration properties were discussed considering their mode shapes. It is seen that, unlike isotropic beams, pure shear mode is not possible for FG beams.
  • Thermo-structural design of a Ceramic Matrix Composite wing leading edge
           for a re-entry vehicle
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Michele Ferraiuolo, Roberto Scigliano, Aniello Riccio, Emanuele Bottone, Marco Rennella The design of the wing leading edge of re-entry vehicles is a very challenging task since severe aerothermal loads are encountered during the re-entry trajectory. Hence, advanced materials and structural concepts need to be adopted to withstand the elevated thermal gradients and stresses. Furthermore, particular attention must be paid to the design of hot areas and connections between hot and cold areas of the structure, where the presence of major thermal gradients associated to significant thermal expansion coefficients variations, can lead to damage onset and failure. In order to face this issues, Ceramic Matrix Composites are generally employed as passive hot structures due of their capability to operate at elevated temperatures retaining acceptable mechanical properties. In the present work a novel thermo-structural concept of an hypersonic wing leading edge is introduced and verified by means of an advanced finite element thermo-structural model.
  • A statistical approach for the fabrication of adaptive pleated fiber
           reinforced plastics
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Moniruddoza Ashir, Jan Hindahl, Andreas Nocke, Chokri Cherif The increasing demand for fiber reinforced plastics for different high-tech lightweight applications requires their continuous development, for example, by means of high functional density. Among various smart materials, fiber reinforced plastics can be functionalized by actuator materials, in particular shape memory alloys. This paper presents a statistical approach to the fabrication of adaptive pleated fiber reinforced plastics. Three geometrical factors – pleat thickness, pleat height and the spacing between two pleats were identified by the half-normal probability plot that affect the deformation of adaptive pleated fiber reinforced plastics during the activation of shape memory alloys. Overall, four responses were evaluated by means of the design of experiment. Significant statistical models were found for the deformation, level loss per cycle, heating and cooling speed of adaptive pleated fiber reinforced plastics. These statistical models were validated through experimental data by the goodness of fit function. The results of the statistical model tended to fit experimental results.
  • Effects of residual oils on the adhesion characteristics of metal-CFRP
           adhesive joints
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Da Som Kwon, Soon Ho Yoon, Hui Yun Hwang Although proper surface treatment is very important in adhesively bonded joints, perfectly clean surfaces have been considered in previous studies. Making the surface of an adherend clean is generally very difficult in real fields. In order to identify the minimum surface treatment that can achieve the required adhesion strength, we investigated the effects of residual oils on the adhesion characteristics of metal-carbon fiber-reinforced epoxy composites adhesive joints. We found that the amount of residual oils on the metal surfaces should be controlled to be less than 1.0 g/m2, and that enough adhesion strength could be obtained if the flame treatment followed. Experimentation also suggested that three or four cycles of ethanol cleansing and five seconds of flame treatment was the minimum surface treatment easily adoptable in industrial fields.
  • Synergistic reinforcement of polyamide-based composites by combination of
           short and continuous carbon fibers via fused filament fabrication
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): Yong Peng, Yiyun Wu, Kui Wang, Guangjun Gao, Said Ahzi The aim of the present work was to study the synergistic reinforcement, by short and continuous carbon fibers, of polyamide-based composites. The three-phase composites were obtained by the fused filament fabrication process where short fiber reinforced polyamide was deposited/printed along with continuous carbon fibers in a layered structure. The properties of the continuous carbon fiber tows and short carbon fiber reinforced polyamide tows were first evaluated by means of morphological, thermal and mechanical tests. The effects of stacking sequence of laminates as well as the effects of both short and continuous fibers’ contents, on the mechanical properties of laminated composites, were carefully analyzed by considering several layering configuration. The results showed that the synergistic reinforcement of laminates by both short and continuous carbon fibers was indeed superior to the individual carbon fiber reinforcement for the tensile strength but not for the elastic modulus. The tensile properties of the laminated composites were higher when the stacked continuous carbon fiber reinforced layers (CCFRLs) were separated. Laminated composites with more interfaces between short carbon fiber reinforced layers (SCFRLs) and SCFRL/CCFRL interfaces showed higher mechanical performance due to the stronger adhesion of these interfaces compared to the interfaces between CCFRLs. The mechanical properties of laminated composites tended to be higher with increasing continuous carbon fiber content. The rule of mixture was used to estimate the mechanical behaviors of short and continuous fiber reinforced composites. All the experimental data was comprised between the upper bound and the lower bound estimates. However, the experimental results were rather close to the upper bound due to the alignment of the continuous carbon fibers.
  • An experimental approach that assesses in-situ micro-scale damage
           mechanisms and fracture toughness in thermoplastic laminates under
           out-of-plane loading
    • Abstract: Publication date: 1 January 2019Source: Composite Structures, Volume 207Author(s): H. Wafai, A. Yudhanto, G. Lubineau, R. Yaldiz, N. Verghese Studying the response of laminated composites under out-of-plane loading routinely involves mechanical tests, such as quasi-static indentation or impact. The phenomenology during these tests is so complex that it is difficult to identify different material properties related to each failure mechanism (damage mode). We aim at providing an experimental approach, which is practical and fast, for assessing the in-situ micro-scale damage mechanism and extracting the fracture toughness in thermoplastic laminates under out-of-plane loading. To this end, we developed a dedicated, micro-scale, three-point bending (micro-3PB) test fitted inside a scanning electron microscope (SEM). In a single experiment, we were able: (i) to assess the initiation of a transverse crack, the transverse crack-to-delamination transition, delamination growth, development of shear-induced microcracks during delamination, and fibrillation, and (ii) to evaluate the effective fracture toughness during transverse cracking and delamination under a representative out-of-plane loading. We used this approach to rank two types of glass fiber-reinforced polypropylene cross-ply laminates, i.e., based on either homopolymer PP (ductile matrix) and copolymer PP (less-ductile matrix), according to their relative fracture parameters. We also performed short edge notch bending (SENB), double cantilever beam (DCB) and end-notch flexure (ENF) to obtain the standard fracture toughness values. We found that the relative fracture toughness values obtained by SENB, DCB and ENF are comparable with that of micro-3PB results. Furthermore, ENF results showed that the delamination process during micro-3PB is dominated by Mode-II fracture.
  • Active Vibration Control of Functionally Graded Piezoelectric Material
    • Abstract: Publication date: Available online 22 September 2018Source: Composite StructuresAuthor(s): Jinqiang Li, Yu Xue, Fengming Li, Yoshihiro Narita The present paper is concerned with active vibration control of functionally graded piezoelectric material (FGPM) plate using the piezoelectric material component as actuator. Using the classical laminated plate theory, the equation of motion for the functionally graded piezoelectric material plate is deduced based on Hamilton's principle and Rayleigh-Ritz method. A velocity feedback control method is used to obtain an effective active damping in the vibration control. The influences of distribution type, volume fraction index and total volume fraction of piezoelectric material on the vibration control of functionally graded piezoelectric material plate are investigated. The calculation results show that the vibration control result is strongly affected by the distribution of piezoelectric materials in functionally graded piezoelectric material plate. It is also found that the total volume fraction, especially the volume fraction index play an important role in the vibration control. Furthermore, under the non-uniform electric field the effect of external voltage position on active vibration control is studied. The results show that one can obtain an excellent control effect by optimizing the structure of FGPM plate and the position of external control voltage.
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